I can't confirm it for sure but I believe plug-ins have to be specifically enabled for AutoUpdate. I could be wrong. Since you have Troi plug-ins working fine on your FileMaker Server, I would assume that you have configured it correctly and put the plug-in in the right place.
In any case, Geoff Coffey wrote a nice article about it all on his Blog:
I would also be contacting the manufacturer (PowerSolutions) direct and asking them. There is an email support link on their web site (www.powersolutions.it).