Motor mover won't lock into engage

That’s one of the most reliable movers I’ve come across and best in the manual version as you have, in my opinion.

Powrtouch have always been helpful when I’ve contacted them.

https://www.powrtouch.com/contact/

Make sure the gap between the rollers and wheel is 19mm. If not adjust as per the manual  if it is 19mm the lubricate the mechanism as per the manual 

We’ve had our fair share of engagement problems from motor movers over the years. Our cure is to spray WD40 over anything and everything that looks like it might move. Then engage and disengage the mover about a dozen time, followed by a further WD40 coat on anything that looks starved.  Finally wipe the surplus from non moving parts. It’s never failed.