There is a series of three gears between the bevel knob and the rack that rotates the head. These aren't user-adjustable, so your best bet is to send it in for repair. If this is happening at specific locations, then you most likely broke a tooth on one of the gears. If it happens at any position, then most likely the last gear is not engaging with the rack. If it is a broken tooth, you may be able to just replace the gear without sending the saw in for service. You would need to contact Festool for their recommendation.
This is an old picture, so you can just barely see the small spur gear at the handle, but it gives you an idea what is beneath the cover.