My bike won't shift from the small chainring to the big one, sometimes it does it after 5 mins of trying. The cables are not very worn and near new and so is the chain, I have tried increasing tension but that hasn't worked. It also shifts very violently from the big chainring to the small one. Please help as I need it to work tomorrow. Thanks Charlie
You'll have to tweak the HI limit screw on the front derailleur, turn conter-clockwise by 1/8 of a turn each time. The best way to do it, is to suspend the bike, shift into low and while turning the crank try to shift into high. If nothing happens, adjust the limit screw as said. If the chain is falling towards the pedal turn the screw back in. (mind your fingers!)
In addition to @Carel's answer to this question, this problem can also be caused by the shifter cable stretching over time.
On most front derailleurs, the shifter cable is used to pull the derailleur cage outwards to shift the chain to the larger outer chainring. Over time, the cable can stretch, so it won't pull the cage as far outward, making it harder and harder to shift the chain to the large chainring. With enough cable stretch, the shifter won't be able to pull the derailleur cage far enough outward for the chain to shift at all.
A stretched derailleur cable can also explain the "violent" shifting from the large to the small chainring - the derailleur cage moves too far inwards, pushing the chain past optimal. The can cause a "violent" noisy shift as the chain jumps around, or it can cause the chain to drop completely inside the inner chainring.
If your front derailleur has an adjustment screw (such as the newer Shimano road front derailleurs), or you have a barrel adjuster in your front shift cable, the quick fix is to simply adjust the derailleur or barrel adjuster to pull your front derailleur a bit further outward.
If your derailleur doesn't have an adjustment screw (such as with older Shimano road derailleurs...) the best fix it probably to replace your front shifter cable - you have to completely detach the front shift cable from the front derailleur and readjust the front derailleur from scratch anyway, and the cable has already stretched to the point shifting has gone bad.