There is a zig zag gear in the top of the machine that breaks somertimes and thus prevents the machine from working correctly. Or it could be the machine hasnt been cleaned and lubricated properly.
Good Luck
Can be several causes. In typical probable order for rotary hook machines;
1.Top thread tension is too high (lower tension)
2.Top thread snagging at spool or elsewhere (for hoizontal spool use larger spool cap and/or machine not threaded per specific instructions)
3.Bent needle or barbed needle point (replace needle)
4.Needle not properly inserted into holder (see specific machine instructions)
5.Debris in shuttle hook/race (clean bobbin, hook, race)
6.Wrong bobbin (replace with proper machine bobbin)
7.Bobbin thread tension too high (reduce bobbin spring pressure, likewise top thread tension)
8.Poor quality or extremely light thread (if using cotton only thread, test sew with a polyester blend and see if problem goes away)
9.Bobbin case (replace bobbin case)
10.Machine timing. (check/adjust timing)
11.Burr in hook race, this can happen if a broken needle point jams in the race (remove burr or replace race)
12.Machine timing. (adjust timing)
13.Hook is worn or damaged (typically not a worn hook unless a few hundred hours of use, look for a barbed or bent hook point)
Watch following Singer video... see testing top thread tension.
https://www.youtube.com/watch?v=O7N4XaIV6ms
and following animation on generic rotary hook sewing https://www.youtube.com/watch?v=zqRvljnNLFk
Do you want to turn the microwave turntable off?
Raise the glass holder, find a circle underneath below, remove that circle, put the glass support back and you will not have the turntable rotated.
But keep in mind that rotation is intended to expose your product uniformly to the warmth, microwaves act like the sun, be evenly tan, have to expose to all parts of the body.
