G'day Mate,
What is the problem with the plane, also what is the plane?
We are running 2 AX's, 1 on a trainer with a 12x6. the other on an aerobatic plane with a 12x4, they both perform faultlessly, the prop size is govered by the type of plane, & the type of flying you are doing.
More pitch means more top speed, up to a point, less pitch usually means, more vertical performance, or acceleration, but less top speed.