Make sure your eccentric screw is adjusted properly. Idgaf what anyone says, a promod is not always the answer for shifting. A lot of the times the shift shaft is worn or eccentric screw is out of adjustment. I had 90+ hp mtrs with 100% stock trans parts shift FLAWLESS under heavy loads.
Another HUGE problem and probably the most common is simply shifter position. Most people tend to have shifter down too far and while shifting the foot is not completely coming off shifter therefore shift shaft wont engage the star for the next shift.