I'm no physical trainer, but some people have a harder time than others putting on weight. I think of players like Jordan McRae, KD, JP Prince, etc. Plus I have a couple buddies I've watched try to put on weight for months to no avail. The human body is a curious beast.
I was hoping for a little more Fulky weight gain as well going into the season. He, Pember and Gaines probably have the same high metabolism/body type that makes adding weight difficult. I had that at one time and then 40 hit.