There are several things that could be getting in the way, up to and including the design of the tail. Moving away from that let's look at a few things. Everything below assumes that all the basic trimming out work has been done over the course of a whole bunch of flights. Things like balance, engine offset, up and down line trimming, etc. Going from harrier to hover requires a quick pull of max elevator, correct rudder input to offset torque and P-factor, and a good blast on the throttle to pull the nose around and push the tail down. Think about wing and tail position when you hit the throttle and you may find the problem. Here's some others.

Consider a c/g that's a little too far forward.

Wing weight imbalance. You need to balance both statically and aerodynamically between the wing halves.

Trim differences between ailerons and or elevator halves.

Wrong type of prop.

Operator unknowingly holding a small amount of aileron. This is a BIG one.

Rudder trim or offset. Incorrect rudder input by operator.