Sprinting Problem on Tacx Neo Smart Bike

Hi guys, 

Love my Tacx Neo Smart Bike so far however I struggle to sprint and get anything close to my typical sprint wattage.
I'm 210lbs and my 5 second power is 1300-1370 Watts

During races I am only getting up to 790 watts and experience the pedal stroke slipping and dropping down to the 6oclock position.

It feels like I am pedaling squares.

Is there a fix or technique? Am I doing something wrong?

Cheers

Mike