ya, they are one of those things that always disappear right after you put them in. You will think they died off but then a few months later you suddenly look at the tank and realize all the aptasia are gone.
These guys are nocturnal and very small, so unless you are looking closely at night with a red light and a magnify glass. you likely wont see them at all. They also take quite a while to populate and spread before they become effective, so just give them time.