I also have that book and have just tried the Caciocavallo recipe and have had the same type of result, the curds did not stretch, even though the instructions did not state it ,I used a digital PH meter and I had a reading of 5.2. I worked the curd for about 20 - 30 mins, but the curd kept breaking/tearing so I took the curds out of the hot water and left them in a collander for another half hour, I then tried a small piece and the curd did not stretch. I have wrapped the curd and placed it in the refrigerator and will try a small piece in a few hours and see what happens.
I read a Kasseri recipe that said to leave the curd overnight then try stretching it, I might try that, or I could break it up even more press it and make Cheddar cheese instead.