Firstly, you need not be in a wait to solve all the questions of a particular difficulty, in order to solve higher rated problems. Given the number of questions you've attempted, 900 rated problems shouldn't be the ones you should be worrying about. If you're unable to solve such a problem, just religiously understand the concept the problem intends to teach, and move ahead.

Try moving up the difficulty ladder, and begin solving higher rated problems, particularly the ones rated >=1200 and try to get most out of each problem you attempted.

Try solving problems on Usaco Guide. You will learn a lot of stuff on it.