I have read the fixes to what appears to be our problem with the top rack of our Kenmore D/W not getting clean. Seems to be either the filter or water inlet value. We're trying the filter first, but can't get the lower spray arm off. There is one large plastic "screw" in the center but it just seems to turn without loosening the spray arm. We've tried using a little bit of force to lift off the spray arm attachment but nothing happens. What are we doing wrong? Thanks.