Jerri's answer Acidity is the key for the stretch in mozzarella. Not enough or too much acid will result in hard curd floating or disintegrating in the water. The pH of your slab needs to be between 5.0 and 5.2 for it to stretch well when kneaded in hot water.

Can mozzarella cheese stretch?

In the manufacture of mozzarella, the curds are then put into hot, salty water. This uncoils the proteins and turns them into long strands, which are then repeatedly compressed and stretched. This forces the strands to line up, creating the famous stringy consistency.

Why is my cheese not separating?

This is often caused by improper temperatures; either you used UHT (Ultra High Temperature) pasteurized milk or your curds got too hot when you were preparing for the stretching stage, or they didn't get hot enough. The temperature of the curds before stretching should be 160° to 170°F.