Some kanji are just sticky. Or maybe the word is slippery. At any rate, if you’re learning kanji you’ll probably agree that while most of them are easy enough, some, whether out of a stubborn unwillingness to fit any memorable story, or some slip of a shade of meaning in the English keyword, or whatever other reason, just refuse to be remembered.
I would compare it to that guy in Zetsubou Sensei who blends into the scenery, but I can’t recall his name. Oh, right, Usui.
To find which ones fell into this class for me, I went to my full list on the RTK site, and sorted by total number of failures. This was the key to getting all of them including the ones I couldn’t even remember forgetting. I took all those that had been failed six or more times (fourteen cards, if you must know) and wrote them on small post-its and stuck them above my monitor in my office. Just the kanji, no keyword or anything else. I sit here for eight hours a day anyway, so if I have them in front of me I will get all the reviews I need to make them stick just by the occasional times my eyes happen to light on them, with no extra effort involved at any time. So far I’ve only done this with one exceptionally recalcitrant kanji but it works like a charm.
Other applications could include selecting two that are frequently confused (like “chestnut” and “horse chestnut” – seriously, Heisig) and sticking them up side by side.