since your problem only exists when none of the T's died, it means that a T that spawned with the bomb dropped it without dying. Just kill him, If he decided to throw away the bomb, then he definatly isn't playing with the team trying to achieve the mission. If he wanted to give the bomb to someone else then he should make sure someone else picks the thing up.
There is nothing worse than 5 T's guarding a bomb point and the stupid bomb carrier camping on the other side of the map (or the idiot with the bomb dropped it somewhere in between and someone has to go fetch it).
If the CT's can defuse the bomb, and all T's are alive, slay the dumbass T who had the bomb last for stupidness.