Combos can only be detonated once per prime. So maybe your teammates are getting their detonators in before you?
Also, once you prime with a specific element, you can't re-prime with that element until it wears off (which usually takes about 10 seconds).
So for example, if you melee/frag/melee/seeker, then your seeker isn't going to detonate a combo because the second melee didn't re-prime.
But you use Ralner's to prime with fire/frag/melee/seeker, then you should get 2 combos because you primed with 2 different elements. (assuming your teammates didn't get detonators in before your frag/seeker.)