It looks as though you might have heads for different gauge spokes. (Possibly 13g, 14g, 15g.)
I have a similar machine with a head for rolling 14g (2.0mm) spokes but when I try to use it I get as many 2 start threads as I get single start.
I read somewhere that you need to chamfer the end of the spoke to stop this happening but it seems to make no difference. What does work is not to cut all the thread off the spoke and feed the existing thread into the rollers. This is OK if you only need to shorten the spoke by say 6mm but if you need to take more than the length of the original thread off it would very tedious.
It could be that the rolling head on mine is knackered with too much slop everywhere.
Does anyone know the secret of consistently producing single start threads?