Mini greyhound won't gain wieght only 3 pounds in 6 months?

We got her from abused case two days before Easter in 2014 we took her to vet she got vaccinated she is healthy now but she is 8 pounds she eats but she is still thin are vet said normal for a mini greyhound she is amazing with are other cats and dogs can someone tell me what you guys think here's abpicture of her vet thinks she will be a year around December she has joint issues but they are better now her bff is a boxer3 and a Pekingese chihuahua 2 then a bassit lab 10 a mini rut shepherd 9 and a min pin beagle 9 and we have 6 rescue cats we even started feeding her by her self are three old figs eat together than are boxer and Pekingese but the mini greyhound don't like eating be herself so we watch we had her on two cups a good to try to get her gain wieght no mean comments. I will post pics in comments they won't load in here