Why do biotic combos seem to sometimes not explode?

If you're playing with a Tech Class, you might be setting off Tech Explosions instead. Overload + Throw or Energy Drain + Throw (in the case of Salarian allies) both result in Tech Explosions. In this case, the overload effect sits "on top of" the warp effect - that is to say, it is suppressed but not removed.

If you throw after warp-ing, the enemy remains blue, but you didn't hear an explosion, chances are you triggered a tech explosion instead. In this case, just keep throw-ing until you get the explosion.

If you're playing with other biotics, however, it is equally likely that one of them detonated your explosion when you weren't looking. If this is what has happened, you will need to reapply warp.