Try either a low residual diet (For example raw) or just the opposite-higher fiber. Also try to feed lower fat...fat is hard to digest and often the culprit of soft stools. Your vet should be recommending a script food at this point. Honestly I would ask to try Hills W/D...comes canned and dry. Feed it solely..no treats or snacks until you can constantly see normal stools for a few feeds then, slowly transition to an otc food. Keep in mind if the w/d helped you may need to find a lower fat, higher fiber otc food. Personally I like the Purina line.