I experienced it too. I passed it off as a game bug, but later in the game I was able to speak with him at a different tavern. It was after I had leveled him all the way so I'm not sure if that's the trigger but you should be able to see this exposition.
I'm at about 85% completion and that liberation contact marker is still there with no assassin there --- "Considering this morning you made a topic on this board titled 'Do chickens have pubes?' I wouldn't exactly say you're Mr. factoid." -MoxRavager
The recruits wander to the various taverns marked by the liberation symbol. I've found two of them at the same Frontier tavern, I've found Boston guys in NYC. Haven't found NY guys in Boston, but I haven't really looked.
As taverns tend to be near fast travel points, just pop to all of them in a row and see who's available where.
Though I have noticed that once you've gotten all of someone's dialogue, they stop going to taverns period.