Check that:
1) The colliding sprites are on the same layer
2) They are members ot fht correct collision class
3) The collision is defined between these classes correctly
4) Non-transparent pixels are overlapping during the collision
5) Neither sprite is set to "Simple (Scripted)"
If all those are true, then I think a collision should be occurring. If you can't figure out why it isn't, I may have to investigate your project. Got a place to post it?