There's a chest that I regularly get gold from and have picked with no problems for some time.

Now I go (with the same pick) and I fail every time, for hundreds of attempts. And I don't learn anything from doing it, even though my advanced lockpick is around 88%, and had learned well from this chest when I failed in the past.

Were there some changes made to particular chests or the skill itself? If I keep trying, I'm going to learn to do it better eventually right?

Seems strange that a lock that I could pick 98% of the time is now impossible to pick and I'm not learning the skill any better after 200 attempts with a 23 int.

I just want to make sure I'm not wasting my time trying.

No idea what you're talking about.