Let's asume your levers are a correct match for the type of brake you have. Also, let's assume that you have a "normal" strength in your hand (say, you can actually lock the wheel in another bike) There are a list of factors that decrease the braking power of a system, some are easier to overcome, I'll try to list them from the easiest to the most difficult/expensive.
Braking surfaces are contaminated
Oil is the most common, specially on rim brakes but can happen to disc brakes too. Dirt and debris can get stuck in rim brake pads an even in some rims. For rim brakes my preferred cleaning method is washing the rim and pads with liquid dishwashing soap, aided with a half worn dish washing pad (the normal ones are usually green, but there are white ones that are softer an designed for anti adherent coated cookware, these can be used new). Be sure to wash the contact surface of the pads.
If the pads have cristalized surface due to excessive heat, they are "too hard" on the surface, so they do not provide enough friction. Cristalized rim brake pads have black and shiny bands in the contact surface. That can be cleaned using medium grit sandpaper (240 to 600). Buff the surface just enough to reveal the still porous rubber underneath.
If there are small dirt particles, metal shreds our other foreign material embedded in the pads, they reduce friction and scratch rim's surface, remove them using a sharp tool, like a thin screwdriver, a spike or a large seewing needle.
Aluminium rims can be cleaned with the same soap and washing pads. If necesary, a slight resurfacing can be done with near 800 grit sandpaper. This is specially useful if rubber has stuck to rim surface (you'd see small black stripes of rubber in the braking surface of the rim). Steel rims, specially the crome plated should be cleaned with the same soap, but use a towel instead of a scrubbing pad. Do not use sandpaper on cromed rims, you'll damage the finish and the rim will start to corrode earlier.
Disc Brakes are less prone to contamination but are still vulnerable. Oil can get in the rotors and be draged to the pads. The rotors can be cleaned with dishwashing liquid ad a towel, or rubbing alcohol and a clean rag. Disc brake pads that become contaminated with oil can be very difficult to clean. Some types can be cleaned with rubbing alcohol, but if the problemm is too aggravated it may be better to change them.
Disc brake pad cristalization is more difficult to identify but the main symptom is a shiny surface (normally the surface is dull) medium fine sandpaper can be used to solve the problem, grit 1000 or approximate. Note that the material of some disc brake pads have metal particles as part of their composition, so small shiny grains are normal in those cases.
Incorrect adjustment
There are so many different types of brakes, and so many different designs among them that I cannot give extensive advice, so my recommendation is to refer to the user's manual if available. Common types of brakes like v-brakes are well known and there is plenty of tutorials available. Most of them (including the ones shown in the OP pictures) can be adjusted using the same instructions, so, almost any adjustment or installatin manual would do.
As for lever reach, travel and position how ever, a badly adjusted lever may give you a hard time. Reach is how far you have to strech your fingers to actually start to pull the lever. Travel is how much you need to move the lever before the brakes engage. Position is wether the lever is actually within the reach zone of your hand. Particularly, travel and position can reduce your ability to actuate the lever, thus your braking power.
Lever travel should be such that the brakes engage when your hand is in the strongest part of the grip. If the travel is too short your fingers are too stretched to pull effectively, if its too long your fingers are too curled or the lever bottoms out against the handlebar. The lever should also be in an angle that doesn't force you to bend your wrist in order to actuate or reach it, the hand should remain in a fairly natural position.
Most levers have small bolt near the handle pivot for reach adjustment. Travel can be adjusted by changing cable tension. Position and angle is adjusted by loosening the fixing bolt, reposition the lever and tighten the fixing bolt.
Old or wrong pads
Aged pads loose flexibility and turn into hard bricks that do not povide friction. Also, there are pads made of a too hard compound with the same consequences. Most brake pads are not expensive, so replacing them should not be too difficult. Finding the best pads is the difficult task, as there are too many options available. You may need to try several options before finding your personal favourite, but once you do, it pays back highly. Do not trust internet reviews to much, as you don't actually know the reviwer or the use he/she tried to do with the items. If possible, ask a fellow rider who uses a similar setup and does a similar type of riding, borrow te bike for a little brake test and observe what pads are installed. Note the make and model of the pads.
Cables in bad condition
When a cable is bent, rusted or dirt inside the casing it causes additional friction. You have to overcome this friction to actuate the brake, so it also lessens your braking power. Cables should be inspected disconnecting them and testing if they slide easily in and out. Any fraying, rusting, casing crushed, bent, worn, is also a bad sign. The most straightforward solution is to replace them.
Stuck pivots
Vbrakes, cantilever brakes, and caliper rim brakes can suffer from being stuck due to rust or dirt and clogged oil or grease. To test, disconect the cable and actuate them directly by hand. They should move smoothly and easily, opposed only slightly by the return spring. Shall this test fail, disassembly, cleaning, lubricating and re-assembly should fix the problem.
Some mechanical disc brakes can suffer from this too. Hydraulic brakes don't.
Bad design / wrong application
Some brake systems are simply not good enough. They don't have the right proportions to give the required mechanical advantage or are made with bad quality materials that flex too esily. On the other hand they may just be fitted to a bike they were not designed for. If you got the bike fully assembld from a reputble dealer, and it is from a reputable maker, this is the less likely case, but if all the tests and possible adjusntments have been made, and yet you cant't get the braking power needed, the only solution may be to change the model, brand or type of brake system.
