I experience exactly same on my ATM EQ fork mount and Astroberry @ RPi 4. I also use OnStep firmware. My RA motor doesn't stop after 30 degs. Should the mount be at the home position first, as is written in the screenshot posted by HY MURVEIT? I didn't try to use the half speed, I'm gonna do it next time.
Because of the fault I used the SharpCap Pro for the PA, but recentrly I've bought an iOptron iPolar camera, which in opposition to the Polemaster doesn't require to rotate the fork at all. After the rough PA using the iOptron I perform PA using a drift method provided by the PHD2, w2hich is possibly most accurate.