Sometimes you'll have much more difficulty with certain items than others. No matter what you do, even after repeated attempts to learn, you just can't get them to stick.

These are "leeches". They burn productive study and review time.

I have a theory that these items have lower adjacency to past experience or knowledge and it's difficult to form mnemonics or other connections. Or they're less novel and don't cause our brain to take interest. That's where all of my leeches lie -- in the realm of things I don't particularly care about.

A good leech management algorithm will back-burner unproductive items so you can focus on the rest of the concept population. There are different types of leeches too -- things you don't get during introduction, or things that you can commit to short term memory but won't stick for long. A good algorithm will identify all of them and block them.

That's really interesting, I'll have to give supermemo a try. I definitely remember those items in WaniKani; a lot of the times they were mnemonics based on pop culture references that I just didn't get, or the mnemonic was just kind of a stretch, or too many similar concepts had been introduced at once. When I stopped I had definitely hit a wall where I just didn't feel like I could keep learning.