How to get glider tow plane to start?

I know I’m supposed to wiggle the rudder a few times to signal tow plane to start. When I do that the guy holding the wing starts waving his arm and the tow plane revs up. But then he stops waving and the plane goes idle again and nothing more happens. No matter how many times I move the rudder (or any other surface) I can’t seem to get off the ground.

What am I missing?

Brakes? I normally cheat and either start in the air or just hit the grave key (the one below escape on a UK KBD) which slews you upwards by a certain amount with each press.

Could also be a mod conflict, test with community folder empty.