Looks like this won't be too tough to figure out, as we all seem to be on the same track here for thinking that the lack of gains are from lots of cardio, not enough "regular" lifting (circuit training is great for keeping the heart rate up and building endurance, but not as beneficial to adding mass), and the likelihood that you'll need more food.
Best thing you can do is start a diet and training log here, update it daily for a few weeks, and there should be no problem seeing exactly what needs to be tweaked to help get things on the right path!
Nothing is going to make you into Charles Atlas except a devotion to living and breathing bodybuilding 24/7 (and for most, the "assistance" from banned substances), so not sure if there's been some reservation about training heavy because of notions that people gain mass easily, or, if that was just a bit of a joke. Just need to check on that one, we've had some people here that actually think that by hitting the weights a few times per week and eating more, they're going to have 22" biceps and will no longer be able to walk through a doorway without having to turn sideways. Nothing in the game of adding mass happens fast, and chances are, once you get a taste of what you're after, you'll be wishing that it came on MUCH faster than the true rate of progress most people experience