1. The wheels are set in a very rigid position and not on a bogie so make sure the are no bumps in your rails that would make the wheels lift of the ground.
2. Add some lego weight bricks above the large wheels in the locomotive if there is room.
3. Check to see if the rubber bands on the wheels are slipping, if they are replace them with some from your own collection.
4. See if your power functions parts are still with battery.
I hope that this helps you. :)