Muscles (or lack of them) question!?

I work out every other day, follow the principles of training (specificity, overload, progression, moderation, reversibility etc) with heavy weights but just right to do my sets which are 10 reps 3 sets per exercise. I also follow a high protein low fat diet but despite this i just don't seem to gain much muscle mass. I have the strength gains but i'm still not very bulky. Any ideas why and how to change this?