Don't feed the shrimps too much. If they are fed too much, they might not want to eat the aiptasia as the shrimps do get stung as well. For larger aiptasias, peppermint shrimps usually tackle them as a group. It takes one shrimp to initiate, after which the rest will follow.
Refer to this thread in '05 for more information.
http://www.sgreefclub.com/forum/index.php?...p;hl=peppermint