Ya, you have to kinda force the trinity in.....the orings on it get caught on the threads sometimes. I found that oiling the threads up a bit helped.
If that's not the problem, then I would ask if your gun is new? If it isn't someone may have altered the treads to fit another reg before you owned it....
Etek 3LT grey/black f/s
Valken Proton greyish/black f/s
68 4500 PE stubby, LP output f/s
68 4500 AIR tank(no reg) f/s
Creed Battery, eyes and noid f/s