There can be multiple issues causing the car to track/pull to one direction, but could be something simple.
These are toy cars but at the same time the amount of speed these cars do for there size, the smallest "bit" that's out of whack will cause this issue. I would do the following:
* Inspect all wheel hub bearings and all (4) drive axles. If you have only 3 wheels applying power this will cause issues.
* De-Tweak the chassis... Loosen all lower/upper bulkhead screws also top deck and give the chassis a slight twist then tighten all screws
* Make certain the rear ball diff is cleaned/lubed and working properly
* Rebuild your shocks with fresh oil (one shock could be dry of fluid)
* Up-Travel/Droop - Make sure you have some up-travel (equal at all 4) wheels. If you have none or very minimal up-travel at one wheel this will create problems. Up-Travel is vital on these cars
Below are two videos that I think you'll find very handy and may possibly assist in solving your problem.
Hope this helps