I'm pretty sure the reason is that the shaft has been assembled incorrectly.
The inner race of the CV usually looks identical on both sides but they're not.
On one side, the hole through which the driveshaft goes has a chamfered edge.
The shaft should be assembled so that the side with the chamfer is on the inside of the CV.
The chamfer is designed to help the snap ring to compress when removal is necessary.
In you find yourself in a situation like you describe, I'd put money on the CV having been assembled incorrectly.
As I'm sure you're aware from doing the other side, the snap ring is a pretty loose fit on the shaft.
Understanding this loose fit is the key to persuading the CV off.
You need to hold the shaft in a vice & try tapping the CV off again.
If you have no luck, you need to slacken the vice off a little & rotate the the entire shaft in the vice grips by 5/10 degrees.
Re-tighten the vice & try again.
If you have no luck, loosen the vice & rotate the shaft by a few degrees again.
What you're aiming to do is to get one end of the snap ring to hang into the groove to enable it to fully compress all the way round.
The snap ring in this (crap) diagram is slightly exaggerated, but you get the idea.
It's a bit like safe cracking!
Every difficult CV I've ever come across has been removed using this method.