The most usual reason for this is that the CG is too far forward.

It could be a wing incidence problem - do you have much elevator trim set to fly s&l? Also, check it in a vertical power off dive, if it pulls to the canopy there is too much incidence.

Assuming that this is ok it is a case of trying to shift backwards as much weight as you can, batteries etc. Once that is done if it still drops its nose inverted all you can do is add weight to the tail.

Best of luck.

James