I know what you mean. Once when I wanted to gain weight I started to eat like crazy. Really, I eat everything what I wanted and even more. It was so easy to spoil myself because everywhere I go, the was a lot of yoummy food like sweets, burgers, crisps, ice creams etc. But after 3 weeks weight not changed so I started to even push myself to eat few snack bars and drink creme. But for next 3 weeks there was nothing, only something like 1kg.
But after without eating more, maybe a little less, I started to blow up. I ballooned 10 kg in one month. After that I slowed it down and not gained more than next 2kg, but gaining is strange sometimes. So keep eating and you will definitely gain a lot of weight