Why My Kitten Does Not Put On Weight??

I have been feeding her the same amount of food with a little more water in a bowl. I use Royal Canin kitten kibble, but my vet suggested that she was not getting enough calcium in it and he gave me some canned salmon to add with her dry food. She is eating well, loves the new food very much and has already started putting on weight.

But every time I give her wet food (she is still young) she vomits up anything ingested by two or three hours after eating it. Also once or twice she did throw up within 2-3 minutes after waking up at night! Why does this happen? What can be done about this? Is there any special diet for kittens that may prevent them from throwing tantrums due to changing their diet? Thank you very much for your response!!