Thezaman,
the mozzarella that this guy makes is very soft and buttery and he told me that you're not going to make mozzarella at 165 F like Polly-O and other companies recommend in their instructions and video, don't worry about the butterfat. This guy has been making mozzarella for 14 years. He showed me the temp of the water in the thermometer! I think that your problem is a combination of frozen curd and low temp of the water.