Seems concave is to tight. Also if very dry, try and slow cylinder speed even lower if possible. Don't take much to knock out dry barley, as feeder chain will do half the threshing.
Open your bottom sieve a bit. Found this to be very sensitive. Opening it up reduces the returns which have a tendancy to wisk their way right back onto the walkers again and straight through the combine. You may have to settle for a few more awns in the sample to increase your ground speed.