Trap Question

LAG_3

Last year this group, especially Steamhead, helped me figure out how to get my 2 pipe steam vapor system working right. With the addition of a vaporstat my problems with gushing from return went away and all baseboard rads generated heat. So I was surprised this winter when I fired up the system, that all the rads heated except one which is the farthest but the second to last branch from one of the 2 mains. Suspecting a trap stuck closed (all traps are in return line in basement), with system cold, I opened the Hoffman 17A and it was retracted and looked fine. I cleaned the trap as best I could but it was pretty clean anyway. Fired up system. Same problem. Then I decided to take top off trap and then fire system to prove that steam could get to the rad and indeed it did. I replaced the trap cap and all was fine. I then made the mistake of opening trap again when system was cool and then closed it. Problem recurred.no steam to rad. I repeated the corrective process above with trap open while steaming and now again it works OK but this time I will leave it alone. It is almost as if that radiator was air bound in some way or needed to be "primed". What would cause this?