How do I make my springer spaniel gain weight?

I had this problem with my Irish setter, if you go down to your pet store and ask them for advice they will give you a good idea, or I would suggest going for a higher calorie content( found in puppy food) or a senior dog food. Also check she is able to eat it well- not spitting out bits... and wetten it with water to make it easily digestable. Make sure it has a high protein content aswell.