Stress, illness, antibiotics or dietary changes can disrupt the balance of bacteria in your dog's gut. This means your dog can't digest food effectively. Your dog might show signs of chronic or recurring diarrhea, disruption after antibiotics, gas or bloating, constipation, irregular or soft stools and other signs.
These are all scenarios when taking probiotics are a good idea, if your dog responds well then it has likely been a gut and probiotic issue.