My three year old Cavalier started getting really soft stool and started pooping more than usual about a month ago. He got put on prescription food by the vet and it seemed to get better, still not firm, but more regular and he was going less. I started introducing Natural Balance slowly, and he went right back to soft stool. This is the third food if tried. Anyone else have this problem? Any suggestions? My vet just says it's trial and error, but it's getting expensive and time consuming.