Only other thing I can think of is excessive friction somewhere between the two transmissions....isn't there supposed to be a bushing between the to counter-rotating gears adjacent to the motors. This is where the vertical up thrust is applied to the main body....where it all happens.
You can assemble virtually the complete system outside the machine and check it......check that the two screws holding the lower head to it's drive "tube" are not causing a pinch to be applied to the inner drive shaft.
I'm flying a cx-2 and the only mod I made is a tail boom.....she's hit the ground a few times but I've never had a problem with her.

Another day above ground.