sounds like your air is high enough. you could close down the top chaffer a bit and open the bottom a hair to give more air to the top. I never care if there are a few empties in the tank, as long as there aren't lots.
If your 1680 is a long sieve machine, close the front part of the chaffer. The long sieves let's you close the front part of the chaffer. This will build more air pressure on the rest of the cleaning system and should solve your problem.