Wifi was down in the park yesterday and thus it was very late in the day before I got on, which is why the short answer...
The Dometic by LG in my house did that too.. I found the following:
The motor was turning (I stuck some plastic over the shaft, put a glass of water inside to absorb the Microwaves and observed rotation of the extension)
The problems were two fold
1: The shaft on the motor was not long enough, and the end had slightly rounded (Nylon shaft) so the tri-arm tha holds the glass turntable popped off easily.
2: The hole in the metal part of the microwave the shaft extends through was too small so the tri-arm could not push down fully onto the shaft.
The solution was 2 parts.
1: I replaced the original motor with a GE part, electrically identical the shaft was brass and SLIGHTLY LONGER, NOTE brass at this point is not a danger.
2: Enlarged the hole JUST till the tri-arm shaft passed through (perhaps 1/64" to 1/32" Larger to allow for expansion)
Been over six years and no problem so far.
In my life I have been very impresed with the quality and design of LG products, Very impressed, and it is NOT a good impression, LG = Lousy Goods in my book.