UWorld usually tests multiple concepts in each question. Ie. what disease, what bacteria causing disease, what drug to treat, what's the MOA of the drug? If you only know 3 of the 4 concepts, you'll miss it. Chances are you never really grasped the concepts in the first place. It's important to be honest with yourself and learn the material more thoroughly on your next pass.
Try doing more qbank questions. That knowledge sticks much better than FA. FA is written in a way to be easily forgotten. But if you do really get knowledge in your head, it's unlikely you would forget it in just 2 weeks.