From the looks of it (but it would be better if you could post the file), the top wall of the recess is a single surface. I would split that one into 3 surfaces - one for the planar middle surface and 2 corner fillets. When you use the ExtrudeCrv command to make this (if that is how this was made), you can set the SplitAtTangents option to yes to automatically make separate surfaces.
[@margaret, The above help file item (both for mac and win) points to Command Behavior settings that are exposed in Serengeti but not Rh5.]
CHEWITS.3dm (731.2 KB)
CHEWITS-wd.3dm (315.7 KB)