its strange you should say training to gain weight. Obviuosly you are female but it would be good to know your stats, ie height, age etc.
fast metbolism is almost a wild card theory, we all burn pretty much our enegies in the same way with the same speed, muscular density however and fat deposits in the body will ofcouse determine how much faster your metabolic rate functions but it is not at such a massive difference you find it hard to put on weight.
The first thought that comes to my mind is you are just not eating enough and training to hard. ie calories burnt = less than calories consumed....
Pass on your average daily eating habits and work out routine and I will be happy to advise where you are going wrong. Without that kind of information people would only be guessing where you go wrong!