These are excellent points. My leeches seem to correlate with lack of reinforcement.
企 is likely to become another leech for me also, as its kanji pronounciation gets only reinforced with a single vocab: 企画. In the same vein, I just failed to remember the kanji pronunciation for 右 because it is pronounced みぎ in all vocabs except one in level 2: 左右 and another one in level… 44. Will I remember it by then?
My strategy for going through the levels as fast as I am able, with manageable batches of lessons/reviews and minimal carryover of items , means that I can usually anticipate how tricky a particular level is going to be. For example, when only 5 kanjis remain locked upon levelling up, it’s clear that learning the radicals will bring quasi no benefit, since most will be useful only in later levels.
Level 17 is especially imbalanced: 4 of the kanjis appear in a quarter of level 17 vocabs!