[EricH]

You don't have enough torque available to the belt. That's what is probably causing the motor to bind up before you apply a little starting pull. If that's what the problem is, your pattern will probably be something like:

Full battery, fresh off the charger, it works.
After a couple minutes, it doesn't work if there's a ball in there, but runs unloaded.
A minute or two after that, it doesn't work without the pull-start.

You're probably going to want to add another 550, or upgrade to a 775 or one of the AM motors.