It’s definitely something I think that most people experience, especially when we do a whole load of lessons at once. The important thing to remember is that this isn’t a race nor is it a test - it’s a learning tool, first and foremost. If items keep getting missed, it simply means that the initial lesson didn’t stick and you need to go back and either spend more time studying the item carefully or else changing the mnemonic so that it’s more relevant to you and thus easier to remember. And just because a level can be completed in less than seven days doesn’t mean it should be. I will often make sure that I don’t do lessons if my apprentice items are over 100 or so. Others will pace themselves by doing 10 or so lessons a day or something.
It’s okay to study between reviews as long as you don’t cram right before you know a session is coming up. I think the best thing you can do is to diversify your studies. When you do grammar studies or go through vocab lists outside of WK, material will often cross over. That kind of overlap is perfect for reinforcing the kanji and the associated vocab in your mind. It won’t target the specific kanji you learned in a particular level, but the more you do it, the more cross-over you’ll see, and thus the better the reinforcement you’ll get over time.
Good luck and happy studying! ^^