Probably a couple of things. It is either bubbling up over the clean grain cross auger and falling down into the fan and/or it is rolling back over the front of the sieve and ending up in the fan. So what do you do? Shoe settings are a big factor. Deep tooth chaffer and sieve help a little. Keep the chaffer as closed as you can and the sieve as open as you can. Not sure what model year you have, but there are some parts to put in the bottom of the front chaffer to make sure all the material is getting back to the sieve and not falling through the black fingers straight into clean grain. Basically, moving material further back utilizes more of the shoe and gets you a little more speed/capacity before grain will end up in the fan. Little things like elevator chain paddle wear, tension, auger wear make a difference too, but in really good yielding crop it is just going to happen.