Check the bearings. Have seen one lock up a few runs in. If they get dirty enough they might lock and spin in the hub and wollow out the plastic from heat. Bearings might be checking out early if your track has a lot of loose dirt. Rear bearings seem to go first on true dirt tracks. Should be easy to spot the problem by inspecting the hubs that seem to move the most.

If wheels are tweaked thats a different problem. Either way, please share what you find.