I agree. Lots of people seem to think that just eating protein will magically make them put on weight. But no, you need plenty of calories too---if you're still in caloric deficiency (which is reasonably likely given that you're not putting on weight) your body won't put on weight. It may even get to the point where it starts trying to burn the protein for energy.(Original post by *NuckingFuts*)
It's a common misconception that the more protein you consume, the more weight you will gain. This is NOT true! ... An increase in dietary carbohydrate, added with a well-balanced diet, will provide you with a higher overall energy intake. Couple this with some weight training, and voila! ... As long as you top up on your carbs - pasta, wholemeal bread etc, and follow a good exercise plan, you will gain muscle and therefore weight. Try and get about 4 resistance training sessions in a week - circuit training is best. Swimming and running, although not classed as resistance based exercise, will give you a good whole body work-out, so keep up those too.
Protein is useful for muscular recovery, and as someone pointed out above can be useful if you're doing lots of weight training sessions. But again, only really if you're eating plenty of carbs to keep up the calories. I used to take a protein/carb recovery shake after big rowing sessions leading up to races. And protein will fill you up, as someone above said, so it won't encourage you to eat enough.
If you increase your training, and you already struggle to keep/put on weight, be prepared to need to eat a LOT A good tip for eating enough if you find enormous plates of pasta too much is to eat slightly less, and often. The protein/carb shakes I mentioned, or energy drinks, or high energy cereal bars (as opposed to the 'diet' low calorie ones) are easy ways to shove some more carbs in before/after training and between meals.