If the adjustment knob is snug, and belt doesn't slip when the handle is firmly in position, AND the handle's axle screw snug... Then, the problem is that the handle's detente, (on the back side where you can't see it), has elongated from use. Remove it to confirm this, then order a new one from Ray Marine
. I have one on order from them now, for my 3000+, and they are currently on BO.
Good luck with it,