Check if your muzzle thread has a relief cut at the end of the thread. If not, check if the suppressor has a relief cut at the start of the threads, so that the threads may go in all the way. The muzzle thread can't be cut all the way to the shoulder due to tool geometry, so there will be fitment issues is neither of the parts have a relief cut on them to make room for the end of the thread.
If you don't have a relief on either, the issue is that the thread cannot go in all the way, as the female thread ID hits the end of the muzzle thread before the shoulder makes contact.
You'll be best off having the relief cut done on one or the other half, so that you get solid metal to metal shoulder contact. Washers, especially crush washers are asking for trouble with suppressors, as a very slight misalignment will cause a baffle strike. A crush washer can get bent unevenly and cause severe issues.
Edit: And do not force the thread in all the way if it doesn't go to the shoulder with finger strength. Galling a muzzle device on a barrel is far from fun, and will be expensive to repair. Usually ends up with rethreading the barrel, and may cause irreversible damage to the suppressor aswell.
