Is the turntable rotating smoothly , does it struggle to move? Check for "flat spots" on the "wheels" of the ring shaped roller or Y shaped roller, whatever sort supports the turntable.
If so you can buy a new ring shaped roller or Y shaped roller( whatever is applicable to your particular brand and model microwave), that the turntable rests on. These come with "wheels", as a complete unit. Unfortunately you can't buy the "wheels" separately.
If it is not the "wheels" it may be the turntable drive motor gears slipping. You have to then buy a new drive motor.