Could be a couple things. She might be sensitive to the protein (usually chicken) in the food - it's more of an intolerance than an actual allergy and is not uncommon although she's a bit young for that to show up. Maybe try a Pro-Plan formula with a different protein base? They have a sensitive skin and stomach one which is made form salmon. Mulder was like you describe for years without us thinking too much about it until we accidentally switched him to a lamb based food off of chicken and everything cleared up.
The other thing is, eating too much can cause soft stools too, so maybe if she really needs that much to keep weight on, find her a higher calorie formula so you can give less? I'm not a big fan of the Savor mix as it contains a lot of soy. We use Pro-Plan sport with my younger dogs - they get the 26/16 version as they have NO trouble putting weight on but in your case I would try the 30/20 one.