If you feel a slipping sensation when using your treadmill, the running belt may need to be tightened. In most cases, the belt has stretched from use, causing it to slip. This is a normal and common adjustment. Make sure that the unit is turned off and that the belt has come to a complete stop.
To eliminate slip, adjust rear roller bolts with the appropriately-sized Allen wrench, tightening both the left and right bolts 1/4 turns as shown below. Try the treadmill again to check for slipping. Repeat if necessary, but never tighten roller bolts more than 1/4 of a turn at a time.