Belt and Pulley

[COLOR=#272A34][FONT=&quot]Hello,
Suppose this pulley is latched to a motor's shaft residing on the floor. On the ceiling there is a similar pulley hanged on a hook. At the center of the belt's height there is an object carried by the belt. When I ran the motor sometimes the belt's teeth does not lay on the pulley's groove which causes the object's position to not be coherent with the motor's rotation. I tightened the belt and the problem still comes from time to time? The motor is moving forward and backward.
is there any idea how to solve this problem?
Thanks
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1. Is the belt is properly tensioned?
2. Is the object's weight isn't enough to cause the belt to slip teeth?
3. Do you have mismatched tooth profiles? There are several different tooth profiles in use around the world for different purposes. Some may even look similar but actually be different. The tooth profile of the pulley and the belt must be the same. Google "synchronous belt tooth profiles" for more information.