Rotary screw VFD issue

Hello.    We have a 75 hp screw comp 

The compressor load valve is working ok but our drive is not ramping up the motor.  Very intermittent.  When pressure drops the load valve shuts and it will build pressure its just that the motor will not speed up     It may do this several times and then it will work fine.   I think there is a problem in the drive or else the drive is not seeing the signal to increase the speed.   Thanks for your help.    Teco invertor in an Eaton 75 hp