If you've already used a particular resource and it hasn't made a difference, I wouldn't recommend going back and doing the same thing again. Each test prep curriculum explains things a little bit differently, so if you've learned all you can from one curriculum, expanding your horizons and studying the explanations from a different set of lessons may help your progress. Instead of going back and re-doing questions you've already done, try studying the rules from some other set of books or online resource, or doing new practice questions. Maybe the explanations from one of those sources will make the rules more understandable or memorable.
Everyone learns differently, so don't be afraid to experiment and see if there are other texts or methods that will work for you!