I had the same problem....we trimmed the hair around his booty....we used a trimmer (may have cut a tiny bit too much off) but it worked like a charm!!!!!
No more poopy butt. I do have another issue though. Has anyone's yorkie eatten his own poop? I didn't know mine did, but he threw it up a bit later after I found a tiny piece of poop on his pad (guessing he ate the rest of it). Ewwww....what can I do?????