Had the same problem once on Fallen Heroes: Countdown.
I took an open tank commander position on Axis, and could not spawn for a while. I also noticed that we already had two tanks (the maximum I think) out and driving around, so I don't think I should have been able to select the class.
Eventually I spawned as infantry in the tank spawn location, but with no weapons, and a tank icon in the bottom left of the HUD (where it normally shows your character or tank status). I ran around the battlefield and even jumped up and down on an enemy T-34 without being shot at, so I must have been invisible.
I hope that is helpful.