I can’t put in a tampon and I don’t know why.

This is kinda urgent for me, since I have water polo tryouts tomorrow and have never successfully put on a tampon. I can always push it in with nearly no problems, but I can never push the plunger to get the tampon out of the barrel, and I have no idea what I’m doing wrong. I’ve tried on multiple occasions, and have tested the plunger when it’s not in me (which worked), so I know nothing is wrong with the tampon. Any tips?