Why did my mozzarella turn out like ricotta?
I have the same problem and went through 3 different brands of milk, thinking they were UHT.  However, after some experimentation I determined what I was doing wrong.  

In my case, after cutting the curd, and while the water was heating back up to 105, we were stirring too much.

The key is very slow gentle movement. Just enough to slightly move the curds, and not disturb them.

"Stirring" will cause you to end up with a nice tasting ricotta, but not mozzarella.