VTOL can't go full throttle in FW mode

Hello guys!

Now I’ve got a problem of my VTOL (tilt rotor): During the ground test I found the four motors work perfectly in MC mode. But when I switch into FW, the throttle goes wrong. The 2 working motors start to spin from 20% throttle and don’t accelerate any more after 50% throttle, which means they can’t go full throttle to provide enough thrust. Ailerons and rudders work fine in FW mode.
Four ESCs and motors are the same and are recalibrated together.

I’m really glad to have your suggestions and helps! Thank you very much!