Well if an ally or escort has a lengthy rescue animation, they will play through the whole thing before following. That's the only thing I can think of.
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It could be two NPC's of the same name. The first one could simply be programmed to stand there and say a few things after being rescued. Later on, you'll come across a boss which you kill that triggers an ally ambush - this new one fights with you.
Try doubling back after the person 'returns'. I have a feeling the other NPC is still there.
There's a very funny are I played, which I believe is called "The Pick-Up-Group from Heck". The idea is that you're playing CoH, and the npc allies are being run by the worst players ever.
At one point, you rescue an ally who says "Phone's ringing -- AFK". Then the character just stands there for several minutes, oblivious to everything, not following or fighting or anything, until suddenly he decides to join the group.
I think it's hilarious, but I don't know how the author did this. Does anyone know?