Wrong! I think of myself as fairly capable, so it pains me to ask, but why is it that the plunger engages the striker but won't stay engaged

I took the 2 pieces off the car, brought them together in hand, and can see them join, but even then won't stay together......whats the deal? Am I missing something here? I did notice the two points that join are not a square notch but more of an angled union (why I don't know) and there was wear on the 2 parts that slide past one another........... I'm stumped
