Add calcium chloride if pasturized.
What does the CaCl do?
The pasteurization process strips out some of the calcium (not sure how but probably the heating and cooling precipitates some out). Not enough calcium results in slower coagulation when you add rennet. No calcium = no coagulation. So you add CaCl to re-balance the calcium in the milk. It'll probably work without but slower. Set times are important when determining how long to leave the curd before cutting so it's best to start out right.
My first Mozzarella didn't turn out great because I used the wrong culture and the curd didn't acidify fast enough. Took too long to get to the stage that it could be pulled.