Hi I have a Eastern German Shepherd. She is 6 years old and she is thin. We feed her twice a day and she sometimes finishes her food and sometimes she doesn't. She got sick the other day after eating her food too fast. She constantly runs... after the kids or in circles. When we brushed her out the other day her tail bone shows and this is very concerning... Why do you think she will not gain weight?

Some dogs are naturally lean.
If after reviewing this chart you feel that your dog is still too thin it would be advisable to rule out any medical issues first through blood and fecal tests and the like.
I would start with making sure is physically sound and then go from there with diet changes.

Edit: the chart didn't load Let me see if I can fix that.

http://www.google.com/imgres?q=dog+w...9,r:1,s:0,i:78