I'd say make sure your front drum is level and equally tightened. Maybe a gap on one side the material slides to the favored side with less pressureIJIJIJ call Harvest Support Joe or Jared 717-371-8222
Check the drum level and also if the elpahnts ears on the front of the rotor. If one is gone u will get on even feeding. Also make sure the rotors are timed
You might also check that both left and right belts on the front are going at the same speed, if one is faster than the other it will throw the bulk of the crop to one side.
what kind of header is itIJ We had a MacDon 960 a couple years ago that did basically the same thing and we never did figure out what the problem was but the next fall we got a 962 with the new adaptor that has the feed drum set back behind the header and has retractable fingers and the only time it backfeeds under a side draper is when you are only feeding one side. We love our 962 now and wouldn't run anything different.