Can you not lift heavier dumbbells on flat bench than you can on incline dumbbell bench?
If you don't have a problem with incline dumbbell bench I don't know why you find flat dumbbell bench a problem. That's pretty interesting.
Maybe it's your back setup and you're not pulling your scaps together when you get down. Maybe it's because when you lie back you don't lie down with good control and you end up starting with the weights in your armpits (happened to me today).
I guess you should get someone to check you out and see whats up.