Sometimes the belt slipping can happen if your timing pulley profile (mostly if it is 3d printed)is not matching the belt teeth profile properly. Sometimes the pulley tooth profile data you get from internet can be wrong. To check this, just wrap the pulley with belt, but dont force it to match, and see if the teeth are going into the pulley groove smoothly, without rubbing the groove side walls. ( I know you found and solved the problem, but just in case....)