I was having this same problem-
Cisco would weewee on his pad, but would never poop on one.
Then it seemed like one day he pooped on the pad, and i praised him to death, and now he almost always (sometimes missing the pad 1/2 ich, but i know his intent was good
poos on the pad.
I don't know what changed his mind, but i know how frustrating it is, so i wish you luck...