Many people start working out at a certain weight and over a course of a year get much bigger and stay the same weight. Body fat percentage is a major factor (not sure if this is your case, just throwing out an idea). Like many others said before, your diet is probably the problem. Sure protein builds muscle but overall calories is what makes them grow and ultimately allows you to gain weight. If you were always skinny, you need to eat, eat, eat!

Another possible factor is your routine. Are you a beginner? How many sets are you doing for each body part? I am guessing not much since you work 3 body parts a day, which is fine if you are a beginner. If you are a little more advanced you should definitely up the sets and if need be add a day or two to your routine.

Hope this helped