I was having the same problem also. I found that if you grip the lock ring loosely and try to turn it, that almost always works.
I have found the same as DannyBoy.
That lock ring is so thin that if you are trying to unscrew it, and gripping it tightly, it actually is flexing and making it tighter against the threads.
I hope that makes
sense.
I very loose grip on the ring and I have had no problems.
When I first got my nemmy, I thought something was wrong with the lock ring, but it was just my massive manly grip I had on the thing.