I had this... Also with transferring some troops to Companions or to castles. It would be blank like above. It would also work in the reverse. I would try to grab a Lord from a prison to ransom them. I could grab 2 but not the 3rd one.
What I think it is is when you get a new prisoner or recruit a prisoner. It takes time for the game to recognize them? (I really am totally guessing).
I just closed the window, exited the town, 'waited' an hour outside the gate (in game time), went back inside and it worked.