If you have any difficulties with your milk setting when making a rennet based cheese (e.g. mozzarella) you should add another 1 scoop of rennet (dissolved into 1/4 cup water), warm the milk to 100°F and allow it to set for a further 30 minutes.
We test all our recipes using fresh whole supermarket milk bought in the UK, but the ways which milk is processed does vary quite a lot around the world. This is particularly an issue in the USA, where a slightly higher amount of rennet may be needed to set the cheese.