You have to switch your main job to monk to level it. Then you can use it as a subjob and it will be at that level (capped at half your main job). Subjobs won't level when they're set as a subjob.

For example, if you switch to monk as your main job and warrior as your subjob, they will both be at level 1. If you level up to level 10 as monk, then your job will be monk 10 / warrior 5. Then if you switch back to warrior, your job will be warrior 18 / monk 9.