Just an update. I completely dropped the Sportmix but I cut the amount of food from 1/2 cup 3x per day down to 1/3 cup 3x per day. Her stool has firmed up a lot but it is still a touch on the soft side towards the end of her bowel movement. I'll keep the food at this volume for a few more days and see if it firms up more. If it does then I'll try increasing the volume back to where it was.
Thanks for all of the suggestions.