Problem with John Deere lawn tractor
 

I just got my LT 166 out, changed the oil, and was checking things
out.

When I disengage the mower, the belt flies off the leftmost pulley on
the deck.

I checked the mower engagement rod setting, and that's fine.
There's no debris in the pulleys.

I called the dealer, and they said it would be very rare for a belt to
be stretched, and that I should check adjustments.

Any advice?