Hello I will help you with your question,
This is quite common and it sounds like the belt tensioner has lost its spring and will need to be replaced and you may have a bearing failing on an idler pulley. Gates has come up with a bulletproof solution that is more expensive than a tensioner and belt replacement but it is new idler pulleys and a special belt that is ribbed on both sides with a tensioner that is as well. This guides the belt at all times so it cannot slip off.
The other thing to check is that the splash shield in the wheel well is still in place to keep water and slush off the belt.
Please let me know how I can help you with this