Mongoose SX 6.5 26" Modified Was originally a MTB but made it a Frankenstein hybrid build. Just made the modifications this weekend and I thought it would go over smoothly. Got parts from a Novara road bike that was due to be scrapped soon because of terminal frame damage. Was working on it as I typed. Don't have pictures because I was too flustered to grab my phone to do so.
Your question needs to include a list of all the parts of the drivetrain you use in order to get a clear and precise answer.
However this is most likely an issue of pull ratio incompatibility, especially if you mixed components from different producers, and moreso road components with MTB parts.
Pull ratio differs between producers (SRAM vs Campagnolo vs Shimano), but also between road and MTB components.
You can find here a detailed explanation of how cable pull and pull ratio works, but also helpful tables of pull ratio of various components: Science Behind the Magic | Drivetrain Compatibility
The shifter determines cable pull. Every time you click your shift lever, the shifter pulls in or releases a certain amount of cable. Different brands and different drivetrain speeds (e.g. 9, 10, 11spd) pull different amounts of cable. For the most part, all the cable pulls are uniform for every shift, with the exception of some of the Campagnolo shifters. For example, a Campagnolo 10-speed shifter pulls 2.5mm of cable five times, 3mm twice and 3.5mm twice.
Derailleur shift ratios, also referred to as actuation, are the amount of movement from side to side of the derailleur relative to the amount of cable pulled. Older Shimano derailleurs all have a shift ratio of 1.7. This means that for every millimeter of cable pulled by the shifter, the derailleur will move 1.7 millimeters.
Based on the facts above and the little clues and pieces of information you provide, I can assume you are using a SRAM shifter with a Shimano derailleur, as it fits your description of the problem.
According to the tables in the link provided above, Shimano road components have a 1.7:1 pull ratio, while SRAM MTB components are designed with 1.1:1 pull ratio.
In your case, when using a Shimano road derailleur with a SRAM MTB shifter (trigger, as you call it), this means for 5 clicks on the lever, 1.7/1.1*5 = 7.72 gears will be shifted
A sixth click would mean 1.7/1.1*6 = 9.27, which translates to the chain leaving the cassette.
The limiters on the derailleur will stop the cable from being pulled further to avoid the chain from leaving the cassette (stopping it from jumping below the smallest cassette ring or above the largest one). This is presumably why you only get 5 clicks out of a 9 step shifter.