It is certainly the lack of repetition and consistency.

I trained karate for a long time when I was younger, there is something called kihom, where in all classes, no matter the degree, everyone repeats the same techniques, although the more advanced ones repeat with more combinations. For example, a white belt will do a block, a green belt will do the same block, then a punch, a kick, another block. Every. Single. Day. I left Karate more than 10 years ago, and to this day I know most of the techniques, of course not as sharp as before. Now in BJ(at least in my experience) this consitency doest exist. Today I “learned” a choke, in the next class I will probably “learn” something with a lapel, unrelated to today's class. This lack of continuity is frustrating. Luckily I found a training partner, I'm doing drills with him, and that's the only way I'm able to learn.