During the first few weeks of using the treadmill, you may need to re-center the running belt. This is possible if, for example, you push off more with your left foot than your right. You must also center the belt after lubrication of the belt. Check this in your manual.

 
Over time, the tension on the treadmill may become too low. This can cause the belt to 'slip': the rollers do turn, but the belt remains stationary. This can lead to the belt no longer turning neatly in the middle. In these cases, the belt must first be realigned and then re-tensioned. This can be done with the supplied Allen key. Tighten the bolts a quarter turn. Prevent the belt from curling, because then it is too tight.

 
Do you hear a grinding noise? Also check whether the belt is properly centered.