Housebreaking a Shih Tzu

I had a shih tzu who loved "cookies". I kept a supply of small pieces close to the door. When he "went" outside, I kept praising him and gave him a small piece of cookie.

It took me about one day to potty train him. When he started using the doggie door, he'd come right to me for his cookie and a hug. Remember the piece of cookie should be small. They really don't care.

I learned in the 11 years I had my darling shih tzu that the breed is stubborn but loves praise and cookies. If you love your little cuddle bug, try cookies and lots of hugs and praise.

Worked for me. Good luck

You might try this positive, no-yell training method: Close up the doggie door for a while and take her out frequently yourself. Have treats handy, and whenever she goes, give her a treat and praise her.

When she goes indoors, let her smell the mess but don't say anything. Calmly take her to a time-out room (without speaking or making eye contact, that will excite her), and put her in time out for at least twenty minutes while you clean up.

Don't speak through the door if she whines. She needs the alone time to make the mental connection you want her to make. It isn't really punishment, she's just not getting the attention dogs crave. Be sure to use the odor-eating spray when you're finished cleaning.

Calmly open the door without saying anything and let her walk out. Now just go about your business. You can stop ignoring her but don't act like anything unusual happened.

Many dogs seem to prefer negative attention to none at all, which is why yelling or saying no loudly (I'm not saying you did) often doesn't work.
Whatever method you use, be sure to be consistent.