Forage wagon bug or something I've done?

[Feel the Gurn]

I can't seem to turn on my forage wagons and the control prompt has disappeared. Does anyone know what has happened here? I am running mods but disabling them doesn't seem to do anything, Cheers

Edit -- I believe i have found the cause, it was related to the "Trailer Fill Limit" setting under Game Settings. The trailer was empty but that didn't seem to matter.