Deej,it's really hard to say specifically but if the probiotic is helping then we have a starting point. Most of the research in probiotic is in the yeast culture area with other bacteria added, there is not a lot of evidence stating many of the other additives in most manufactured probiotic are helpful.
I would consider looking into EquiShure by Kentucky Equine Research, it is a great product as a digestive aid in the hind gut and the only product out currently that is buffered to release in the hind gut. Take a look at this link to an article from thehorse.com, it's worth reading and can answer a lot od questions.
thehorse.com article EquiShure