Our first guess would be one of the drive belts needs to be adjusted (or replaced). We are including links to our Operator's and Parts manuals.
You'll find how to adjust the transmission drive belt (do this first), then check the tension on the wheels drive belts. If they are too loose, you'll need to replace them because the wheel drive belts are spring tensioned and there isn't an adjustment procedure.
Hope this help you out and if you have any other questions, please let us know by giving us a call at 1-800-667-5296.