So irregular bleeding, exactly how you're describing is probably the top side effect of IUDs. So it's exactly what many doctors would expect to happen.
The effectiveness of the hormones in suppressing that bleeding is actually likely to get better over time, not 'wear out'. It can take a while but it often improves.
The IUD even reduces the pregnancy risk without the hormonal element. Just by being there, the uterus 'reacts' and cervical mucus becomes anti-sperm. The hormones are really helpful too of course, but their effect on bleeding, or the existence of bleeding, is a little seperate from how they prevent pregnancy, and doesn't really tell you any useful information about how well it's working as birth control. It works in a number of ways which is probably why it's such a successful form of contraception.
"In between two tall mountains there's a place they call lonesome.
Don't see why they call it lonesome.
I'm never lonesome when I go there." Connie Converse - Talkin' Like You