I have similar problem...
17 years old, 60 kg. I'm quite an active person, pretty long walks almost everyday (nature photography, swimming, rock climbing etc).
Yet for some reason, I can barely get over 60kg. Highest I've went was 64kg, but than got pretty bad flue and went back to 60... since then I can't gain.
I've never eaten meat in my life (religion), which I'm sure is a big factor, but noone else in my family (2 other brothers) seem to have or had the problem.
The reason I say it's a big factor is because, I find it sometimes hard to find snacks around the house that motivate me enough/worth my while to get up and make. A lot of times I just feel like going out and wasting money at some take away.
I'm yet to try protein drinks, do they really help a lot?
Any advice much appretiated... at my age it brings down the confidence in myself and makes it difficult to really enjoy life, like I should be at the moment.
Greg