Weight gain

I'm under weight and very skinny. I tried all means to gain some weight but they always fail. Even went for check up and everything is fine, and was told as long as I'm healthy, that's all that should matter. However I want to gain some more. I'm 26 but usually get mistaken for a teenager because of my size and baby face. Aside from that, I have a big appetite so eating is not a problem since I usually eat healthy foods recommended for weight gain but there's no change. The past months I've been getting enough sleep and barely have any stress.

Is there any other recommendation I could try out?