I have the same body type as you. If I don't eat often, I'll lose weight very easily. So I know the struggles it takes to even get a little bit bigger!

You have to eat like every 4 hours, and a lot of food as well. Just don't eat things that are crappy, but anything goes. Just eat!

If you can't find the time to get food, that's when protein shakes are useful, as Boobs say, as a meal replacement.