We've had vets tell us it was due to partially undigested food left in the stool. We've had vets tell us it was due to a dietary or hormonal imbalance. We've even had vets tell us it was a learned behavior that arose because the breeder left the pup in with the mother well past the age at which the pup should have been weaned inviting the pup to "learn" this behavior from its dam.
Whatever the reason, it is a challenge that can be overcome through training and positive reinforcement. If you must, for now, watch the the dog closely and as soon as it "performs" immediately remove the stool - replace it with a toy or a treat and praise the dog for playing with the toy or taking the treat. I know you can't do this forever and we have tried dietary additives, vitamins, etc. along with the supervision and eventually we achieved the results we were seeking.
Follow up on the advice of others here and seek out professional advice -
just don't be suprised when you end up running through a number of treatment
alternatives. Keep us posted - this IS a common problem and you may be able to help
someone else along the way.
Larry Divine Maltese