It's a fact of life when you have Shih-Tzus - they get mats. It doesn't matter how often you wash them or brush them, they're going to have mats. So what do you do?
I used to try brushing them out but this is very time consuming and sometimes doesn't really work no matter how hard you try. I did go to the pet store and pay $18 for a little rounded contraption called a "mat remover" that you place in the mat and slowly work up through the fur...the little razor-like insides are supposed to cut through the mat. It works better than a brush, but it's still time consuming and we all know what it's like to get our Shih-Tzus to sit still while we're doing this...next to impossible.
This evening I gave the boys a bath. Little Rocki has some mats...his hair is longer since I didn't have him cut after he got out of the hospital (when he had Parvo) because they had to shave some areas where he had blood tests and the intravenous and I wanted to wait until his scabs were gone and he was completely healed. But when he was partially dry I noticed that the matted parts stuck out from the damp areas. So I called him over and got out his little brush and started brushing the damp mats. They started coming apart much easier than when he is dry. Not only can you spot them, but the fur is more pliable when it's damp. He had one behind the ear (a common area) and one on his back and I got them out quite easily.
So there's the secret...wash your dog...let them dry a bit...and then gently brush out the mats.