The regulator on my waggon is screw down it has a coarse pitch and rarely  needs more than half a turn for max speed or pulling passengers.  It is activated in a threaded guide with a gland nut on the body with the usual seal to allow self alignment .

The live steam is on the top of the cone which tends to push it down onto the seat .   it never leaks ( I think)! The thread was cut loose to ensure the cone could have a trace of movement to get rid of any possible misalignment
so,

1 is the hole clean on the arris?

2 is there enough slack in the thread

3 is the thread for the screw true to the body of the regulator

4 obviously the cone needs a good finish

hope that helps, some investigation needed, but it all depending on the exact   layout in your arrangement   My total screw, gland and handle  is only about 1.5” high. it’s difficult to help without more detail