My test scores are going down

I’ve taken 4 diagnostic mbe tests with Quimbee - in chronological order my scores have been 73/100, 71/100, 65/100 and 63/100 (today). I am so discouraged. I’m spending so much time studying, reviewing every question, making flashcards etc. I don’t know what to do - I still have half of Quimbee’s course to get through so I can’t spend all my study time reviewing, or is that what I should do? Anyone else been in the same boat, anything you’ve done that worked?