You may also want to put a bit of grease on the actual threads before you install the back cap.
One trick I also like to use is to press the back into the threads gently, then turn backwards about a half turn to make sure I have the threads lined up properly, then screw it in. If you encounter a lot of resistance then something is wrong.
The reason I do this is because the threads are sharp so if you cross them even a bit it can cut an oring. You want it(the cap) to go in smoothly on it's own and not force it too much.
DM14 grey/copper F/S
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