Test pulley set screws, making sure they are tightened all they way
Re-tighten if loose. If this occurs often, consider using thread locker (such as Loctite) to prevent the set screws from coming un-done.
Test belt tension
Follow the belt assembly instructions to set the belt tension. Belts should be taut but not overly so.
Grip the belt near the clip and pull away from it, testing if the meshed teeth hold together or slip apart. The teeth should hold together.
Wrap the belt (near the clip) with a zip tie, heat-shrink, or electrical tape to prevent belt slippage.