neighbor had same trouble with his last year. Only seemed to do it on the bearded wheat. Worked on getting the auger fingers right set the header auger close to floor. etc.   His was feeding in bunches then back feeding around the throat chain.  Lot of things need to be exactly right. From good rasp bars clear to the front.Might need to open concave some to allow it all through. He was even cutting half a header width and it would still do it.

  Will ask neighbor if any one thing helped the most.