Since @AndreaColombo is posting @devSin's laundry list of trigger/action fixes, I'd add one of my own: IfValidForPartyDialogue. The trigger seems to randomly fail, seemingly due to line-of-sight issues.
The classic issue most people experience with it is Irenicus' reveal at Spellhold, when he fails to recognize Yoshimo in the party due to IVFPD failing.
Sorry I can't be more specific, as there doesn't seem to be a consistent way to replicate this. A lot of modders just avoid the trigger by using InParty+InMyArea instead.