There's a couple of things that might help here, any one of which will probably help on its own...
1) keep track of which kana I'm consistantly missing and let me see which they are afterwards if I choose to. Perhaps being able to see a kana chart with the ones I missed highlighted, so I know which ones I need to review.
2) put some brains into the ordering during the quizzes, and if I consistantly get a particular one right, decrease the frequency that it appears, and increase the frequency on ones that I get wrong consistantly.
3) maybe a corollary of #1 above that lets me adjust the kana set used in the quiz to include only the ones I missed on my last quiz.
Just throwing out some ideas of things I think would be helpful with it.

Thanks for the great work guys, this place is awesome.
