This is my second umbrella, as the first one wouldn't rotate AND the large screw-in support that adjusts the angle of the umbrella was completely stripped and unreplaceable.  (Bravo Wayfair for immediately replacing it).  When second umbrella wouldn't rotate, I examined the base and realized that it's fit with screws that are too long.  When tightened, they dig into the layer below and prevent rotation.  Went to the hardware store and put and extra 2 washers on each one, and it now works like a charm!