The trick is getting the right species. Peppermint Shrimp is a common name for a couple of different kinds of shrimp.
You want to make sure you have Lysmata wurdemanni which are Caribbean.
Beyond that, don't over-feed prepared foods and they'll eat the Aiptasia. If you are over feeding prepared foods, the shrimp won't need to scavenge the Aiptasia - and the Aiptasia get fatter from all the random food bits they catch.
Jenn