It should be a pretty tight fit but nut is slightly bigger than the compression ring/sleeve so should not have to be forced onto the pipe. Sounds like the pipe could have been slightly flared or out of round. They do make tools for chamfering the edge of the pipe and getting it back to a round shape. Or you could just use Sharkbite fittings.
Sharkbites will fit more readily than the compression valve if the pipe is slightly out of round?
Just ‘sweated on’ three Sharkbite 3/8 angle stop supply valves. ALL THREE LEAKED AT THE BALL VALVE (not at the soldered joint)
I replaced with Brasscraft.
Called SharkBite. Customer Service said “What Do You Want Us To Do About It?”
Thank You SharkBite !