My 2-year old was thin too. They are just so energetic they burn it off faster than they can put it on. I started putting a raw egg in her food. I heard here that putting the shell in is also good for joints, but Sadie just spits it out. Texas Belle (on the forum) has a good recipe for some treats that put weight on fast (if you show the dog), but usually just time, and healthy, high protein and calories gets the dog there. I would tell you I was nervous about Sadie's weight at a year and a half, but am comfortable now at 2 1/4 years.
How much does she weigh (and/or post a picture).