Currently, if you give a wrong answer, you get a red bar. But then, if you answer the item correctly in the same session, you get the negative feedback of that item’s level dropping.
My suggestion is to move the ‘level down’ indication to happen at the first wrong answer instead of at the correct answer. I can’t be sure, but I think that seeing the negative feedback on correct answer is causing me to make more mistakes by associating the correct answer with “red, wrong” feedback.