Definitely not an expert when it comes to Mozzarella but I believe that it comes out "rubbery" if you overheat the curds. You say you are using the "30 minute" recipe. Does that mean that you are heating the curds in the microwave? You may be overheating them. As soon as the cheese is shiny and has come together I think you need to stop the heating.
it is farm fresh milk.basically the recipe is 1 1/2 tsp citric acid in 1 cup water add to the milk and heat to 90 degress,take off heat add 1/2 tsp renet added to 1/4 cup water to the milk, cover and let sit 5 min. then cut the curds put back on heat to 105 degrees.let sit 5 mins then drain curds.then add salt to whey and heat to 180 add to curd and fold and stretch.