It is very difficult for some people to gain muscle mass and very easy for others. Is it fair? No, it isn't but genetics has a lot to do with most mass gains when you work out. However, I find it weird that you are working to intensity and then picking up lighter weights and then doing them not at full intensity. You should always work at full intensity, that's why your not feeling the typical burn when you are done your workout. Secondly I would try to take creatine, I found Cell-tech to work very well for me.