I've been trying to gain weight for about 4 years now, and pretty much havent gained an ounce. 6' and 160lbs has been my profile for far too long, and this summer I made a commitment to bulk up (moving into a new apartment which includes a gym helped out).
So, three months after starting a pretty consistent routine of working out three days a week, upping the weight every week for heavy exercises (ex. Curls went from 25 x 8 to 40 x 8), and upping the reps for pull ups, push ups, and crunches alike, I definitely LOOK bigger and AM stronger, but weigh the same...
I don't think its meant to be.
Diet may have had something to do with it, as I often didnt find the time to eat three times a day, but I always got adequate protein via whey.