I have a few pickups in a game of mine that...disappear. I have them set up in the scene, and you can see them in the game right away. They disappear (almost) immediately, though, and are removed from the hierarchy in the middle of game play. The two pickups have separate scripts:
Edit: changed name of collision.
The question has been closed Jun 11 '12 at 11:19 PM by You! for the following reason:
Problem resolved without definite reason.
[WRONG : The objects are removed from the hierarchy]
The way you handle the collision is a little confusing, what is MainM ? Anyway, your pickups are probably going through the ground. By modifying the position.y, you're only modifying the matrix, not the physic, so it won't collide with the ground. Use a rigidbody, the gravity is applied by default.
Ok, second shot : use prints before or after the Destroy. My guess is there is a collision at the start, you need to figure out why and with what. If it still wrong, I'll move my answer to the comments so you can be unanswered.