Mac and Cheese - Fail!

I have been wanting macaroni salad for a few days and decided to make some last night. I had already started cooking the pasta when I realized I didn't have all the stuff to make it, one day perhaps I will learn to get all the ingredients together before I start cooking. So I figured since the noodles were already cooked I would make some macaroni and cheese. I had a recipe bookmarked to try and I followed the instructions except I used a cup of mozzarella and the rest cheddar. I don't know what happened but the texture of the sauce was gritty. Was it the mozzarella? Should I have cooked the flour longer? Total failure :-(