This may be a result of the drive belt tension in your unit being too loose. Follow the instructions in your assembly instructions to remove the top cover.
Once the top cover is off, try the following to fix a loose drive belt:
- Tighten down the belt tensioner bolt on the left hand side of the brake mount.
- Turn the belt one full rotation.
- Place the cover back on top of the machine and try the machine. If it still feels loose continue making these adjustments until the machine slipping disappears.