Check the thrust line alightment for up or down angle with reference to the wing.

But it's quite probably that it's nicely 0-0-0 for motor wing and tail. So that leaves you with the idea that you may have your CG a little too far back. Do a search on "dive test" or "dive testing" to find a write up on how to use diving to determine if your CG is on the neutral point or in front or behind it. I'll bet you a bunch of banannas that you've got your CG too far back.