I have made mozzarella several times and each time I have improved the process. The quality of the milk, and the type of rennet are important. After a fair amount of research, I found www.cheeseandyogurtmaking.com
and bought liquid animal rennet and citric acid from them along with a book that has been quite helpful.
I have been trying to perfect the mozzarella before moving on to other cheeses, but am itching to try cheddar.