This is a tough one... not sure I have any advice.
My Biggie hardly ever poops inside. He does his business outside most of the time. He only uses his pad when he really has to go when we are at work and 99% of the time he just pees. On the rare occassion he does poop on the pad sometimes the poop drops just off the pad. I notice when he poops outside that he kinda moves a little bit right after he squats..I figure he does the same when using the pad and just missed since it is such a small area. Hang in there....