Ok, thanks. Let me provide some additional info.
I am using the optional table top bracket on one unit, and the standard flush mount bracket on the other. The table top mounted unit has the more sever light spots in the lower corners, the Flush mount has the same, but less prominent. These light spots only have developed only recently.
The more severe unit, the table top, has the side knobs that are somewhat tight, but any looser and the mount won't hold position, The teeth on the bracket that engage with the unit are not very aggressive. Also, the unit has side knobs for mounting, the light spot is in the lower corners. As I mentioned, the flush mount unit has the same issue in the same areas, the lower corners.