Why won't a 5/8" compression fitting with a ferrule integrated in the nut fit onto 1/2” copper pipe?
Pipe and tube are measured differently.
pipe is measured by inside diameter and tubing is measured by outside diameter. Depending on the fitting you may have the wrong one. I had a heck of a time remembering this when I went through my HVAC training as plumbing and electrical are normally pipe and hvac is normally tubing, back then the web was not as helpful as it is today for a full explanation and the 1/8” differences I believe that is your problem , look up pipe verses tubing measurements.