Our drivetrain mysteriously started drifting to the right 2 days ago despite no changes being made to the lower part of the robot before then. We tried grinding down the plastic base to make sure it wasn’t because it may of being hitting the ground but that didn’t change anything despite it completely not touching the tiles. We checked for possible beam tension and looked for any delays in the wheels when spinning but found nothing wrong. We made sure our motors were all tightened up and we shaved down our gear teeth a bit to make sure it wasn’t hitting the ground. Even though its a slight drift, it affects coding and its making it much harder than it should be, and we only have a week till we leave for worlds. My team and I are getting hung up about it and we’re on a tight schedule. Please help ASAP 
Try cleaning your wheels or adding a PID controller.
if your drift is affecting turning and not panning you can just use a simple inertial sensor to track and get back to the correct angle. If you are panning you can either use odometry to help correct the movement or install traction wheels. also, try checking if your drive base has equal friction on both sides. In my opinion a drive base with mediocre friction on both sides is better than having bad friction on one side and amazing friction on the other. If you don’t want to do those things you might be cooked.
Post photos of your robot so people can see what you’re dealing with. All you’ll get are guesses until you do that.
are the wheels on the slow side slipping, or are they just slow to accelerate? also, it would be helpful to know if it’s doing this only under acceleration, near top speed, or both.