Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear about Ruby's increased frequency of large stools.
Are they soft?
Large, loose, frequent stools indicate changes in motility of the gut. It can also lead to bacterial overgrowth in the gut. Probiotics such as Fortiflora or Benebac can help replace appropriate bacteria and may be beneficial for her.
I assume that she has had stool samples checked to make sure that parasites aren't part of her problem. Have any other diagnostic tests been checked?
Has she had a fecal culture to check for abnormal bacteria such as clostridia?
Is she losing weight?
It is quite possible that she has a food allergy/sensitivity or inflammatory bowel disease and that she needs a different low residue, easy to digest food or a hypoallergenic food to be able to properly digest and absorb her food and not have large, frequent stools.
I highly recommend a trial of a sensitive stomach diet. Both Science diet and Royal Canin make these foods.
If those diets aren't enough to improve her condition then you can try prescription foods, either Hills i/d or Purina Veterinary Diets EN. No treats, table food or edible chewies while she is on her food trial. If she does well she can eat these foods for life as they are balanced. Having had 2 dogs with inflammatory bowel disease I have a personal preference for Purina Veterinary Diets EN. Dogs with food allergies can benefit from Hills z/d or Purina Veterinary Diets HA.
Dogs with inflammatory bowel disease will worsen with stressful situations. There may be times when she will need medications too, such as metronidazole or even steroids if that is her problem, but I have found that a consistent, easy to digest diet is very helpful for long term control.
There are other possibilities too.
Addison's disease, which is a poorly functioning adrenal gland, can lead to chronic loose stools and vomiting. These dogs cannot handle stress at all because their adrenal gland doesn't produce cortisone when stressed and their electrolytes can be off too if their adrenal gland isn't controlling that normally either. We see vomiting, diarrhea and sometimes physical collapse in severely affected dogs. Testing is an ACTH response test to check adrenal gland function and checking electrolyte levels. Treatment is steroid replacement therapy and electrolyte replacement. This seems less likely with her as she seems happy and active otherwise.
Pancreatic insufficiency is another possibility. These dogs have a pancreas that produces a decreased amount of digestive enzymes, and the amount produced can wax and wane in some cases, especially early in the disease process. Testing is by running a blood test called a TLI which checks for digestive enzymes. Treatment is replacement of digestive enzymes at each meal. An easier to digest food would be expected to create less problems with digestion and as such less stools too.
Kidney and liver disease can cause loose stools too, and possibly more stools, but I would expect her to be sicker in general and this doesn't seem to fit with her.
In short it sounds like a diet change and some diagnostic testing needs to be done. They can be as simple as fecal checks and cultures, as well as a complete blood count and biochemistry profile to assess general health, and a TLI blood test to check pancreatic function. Or they can be more invasive such as biopsies of her gastrointestinal tract to look for inflammatory bowel disease or infiltrative cancers such as lymphoma.
Please let me know if you have any further questions.