Everything is working for me on the latest Maxwell Studio version. I often have to re-render parts of images as the SketchUp plugin unfortunately does not have this feature.
Are you on Windows, or Mac? I wonder if that makes a difference...? Are you sure you are selecting the region in the 3D model window so that the selected area is lighter than the rest? Sometimes I have found it can be fussy as to whether the area is actually selected. The selection area doesn't work in the FIRE window, only the 3D model view. Also, it seems that FIRE does not respect the selection area either, so a full render needs to be done.
I didn't test it, but maybe there is also some functionality difference between GPU and CPU renders. You didn't give much info. CPU should work fine.