Hmm, that's odd. Is there a noticable height difference between the top of the plinth and the top of the motor case? The motor should be no more than about 3mm below the top of the plinth. If the distance is greater, (and the feet are properly installed on the motor case and the cones are square against the plinth), try raising the motor.
The belt is self correcting if placed unevenly around the platter. Even so, when it's completely level it should sit approx 1/3 of the way down. Is this the case?
FWIW-I've never had a problem with my belt.