There may be something mechanical wrong inside the fitting like a displaced "O" ring or something.
I always coat any type of refrigeration connection in refrigeration oil before I try to seat it or tighten it. If I had to lubricate it and had no refrigeration oil, I'd likely use a dab of synthetic motor oil or even vaseline, sparingly. Sometimes you have to lube tight "O" rings.
I'd inspect the male end of the connection for burrs, sometimes when they trim the end of metal tubing (even just a nipple) the cutting wheel leaves a ridge at the very end. You have to trim the ridge or de burr it or it can ding or cut the "O" ring.
That ridge you sometimes find at the end of a piece of tubing is just a hunch, but likely the first place I'd look if the connection gave me trouble.
I'm not sure about the configuration you are working with, but I have seen some types with a raised locking ring on a male type end that goes into the connector. The locking ring is sometimes too large a diameter and makes it really hard to get the connector together. It is usually just a few thousandths too large, a little sanding might help.
Just some guesses that may help.