People training for many years can often, should they want to, gain 20 lbs, but that's not going to be 20 lbs of muscle. It will be mostly fat. Indeed, for a very advanced person who makes a major push in training for four to six months, the loss of some body fat and a gain of several pounds of muscle mass beyond a level achieved in the last four to five years is a terrific accomplishment. If you can produce such a reasonable transformation, don't be disappointed. Be proud of what you've been able to do because, in fact, it is an excellent outcome.
Expectations have to be grounded in reality.
Frustration and disappointment are often rooted in purely fantasy-based expectations. Couple some unrealistic expectations with some unsubstantiated beliefs about training intensity, and then you have the makings of some major problems. Besides never questioning if this was actually true in fact and not just in theory, many of us never wondered if just like too much of anything, too much intensity could lead to poorer results than some more reasonable level of intensity. Again, take Lance Armstrong, a person at the absolute extreme end of the genetic continuum even for his sport. Notice that I am not saying that the stimuli for strength increases and hypertrophy are completely different.