If the barrel is already properly bottom tapped, you will also need to either counterbore or countersink the rear face of the female thread in the breach. Most likely what is happening is that the male threads are not turned all the way to the vertical face of the breech plug, which is normal. That small unthreaded portion of the male thread is probably jamming on the face of the female threads at the open end of the barrel breech.
By adding a short counterbore or countersink on the female thread, you should provide the thread relief necessary for the plug to turn all the way up to the barrel. Hope that makes sense.
Or if you had a 3/4-16 die that has very little thread lead in on one side of it, you might be able gain another 1/4-1/2 turn by running the plug into the die up to it's shoulder. Use plenty of thread cutting lube. The problem with many dies is that they have excessive lead in taper on both sides, so you might not gain anything if that is the case.
The other way to do it would be to turn a small thread relief groove at the rearmost portion of the male thread where it meets the vertical face of the plug. That removes the unthreaded portion of the male thread. I wouldn't recommend that with a Hawken plug though (if it is has a percussion snail). Usually the flash channel is drilled at an angle and passes very close to the corner where the male thread journal meets the plug face. Cutting a thread relief groove in the plug could intersect the flash hole.
