Ava is a cutie pie! As long as her ribs and spine are not sticking out, I wouldn't worry. She looks good to me. Reminds me of when Teddy was a puppy. He was very skinny, but filled out a little in his third year.
Here is a picture that shows a little of how thin he was, and he had already put on a little weight:
Wordless Wednesday: Bath Time Blues - Dog Life and Style
I had to do a lot of handfeeding with Max the first year. Now I have to watch how much he eats.