IME a few different factors influence the effectiveness of peppermint shrimp on aiptasia:
1) make sure you have the right species. You probably have seen this picture circulating in the forums. The common believe is that the first one "wurdemanni" is the true peppermint and best chance to eat aiptasia. The rest are imposters. HOWEVER, I recently got D (boggessi), which also ate aiptasia in my tank. This is what I currently have. In fact, algaebarn seem to be marketing these boggessi ones as aiptasia-eating peppermint so may be now these are good too.
2) a few years ago when i had the wurdemanni, it was more brave, they roamed my tank actively hunting aiptasia. Now I have boggessi, they are very shy and always hid behind my overflow box. In fact I never actually saw them eat aip, but I have noticed the aiptasia disappearing, starting from the rock closest to the overflow box
3) they maybe territorial - or afraid to go into other species territories - right now I have a copperband shrimp claiming another rock, the aiptasia thats on that rock never seemed to be eaten
4) if there's enough to eat they will not venture too far out of their home to hunt aiptasia - right now it seems in my tank there is an invisible border on how far out away from the overflow box is now the "aiptasia free" zone. My tank has mature rocks with a lot of pods and worms. During the day time I see the peppermint shrimps picking at things next to the overflow box so it is also eating at other things so they don't need to venture too far out.