Hi Mike. Sorry to hear youre having problems. This is likely to be a 'colitis', which is inflammation of the large intestine. When the bowel is inflamed, it leaks, producing blood and mucus - this is the 'jelly' you describe. If this is an ongoing problem, and no infectious or parasitic cause is found, it is likely to be due to a food allergy or intolerance.
When you say you have her on a 'sensitive stomach' food, is this a prescription diet? Most commonly the dog is sensitive to the protein component in the diet. A lot of the time this is chicken, as this is present in 90% of dog foods. You may need to put her on to a diet made from a protein that she has not been exposed to before. In the UK we often use food made from duck, caplin, catfish etc as the dog is unlikely to have developed any sensitivity. You may need to ask your vet what sensitivity diets are available - but i get the impression this may have already been done.
In some cases, I'm afraid that dogs need long term medication- a combination of antibiotics, steroids, or other immunosuppressants. I certainly see some dogs that have intermittent bouts of colitis through their lives, but a lot of them can be controlled with strict diets. If things do not improve, then in some cases gut biopsies are appropriate, but there are risks associated with this so I tend to reserve this for my worst cases.
I hope this helps. Please let me know if you require more information.
Dog Veterinarian
Small animal vet in UK for 8 years, Cambridge University. Special interest in diagnostic imaging.