The direction of the thread on the bolt and the inside of the head are the wrong one.
If it spins to the right past 1200 position as you look down the shaft at the back of the speed head, it should be tightening the speed head onto the bolt.
The turning of the shaft should be forcing the bolt into the speed head.
Conversely, if the shaft and speed head spin the other direction when operating, the thread has to be opposite the first example.
Anything the speed head does, hitting weeds, should help tighten it.
Unless some one else has a better idea.