Possibly you are letting more crap through the coarser cake that is actually clogging your screens. Maybe you should try precoating with the fine stuff and then body-feeding with the coarser stuff. I have had similar results to what you are describing. IMHO too coarse a filtrate will lead to crappy filtration runs. I'm hardly an expert but I have done what you have done with similar results. Just my 2 cents.