Welcome Chili to the frustrating world of mozz making. My first batch didn't work out, then I had great success with the next half dozen batches, even taught a friend to make it, then the next 4 batches, including yesterdays did not turn out.
I get two different results. One is the same as yours, where the curd doesn't want to stretch, and goes grainy and crumbles no matter what you do, and the other is where I get a partial stretch but loose all the butter fat in the hot water, resulting in a dry cheese, (that happened with the last provolone I made).
Yesterday I checked PH and even though I left it an hour longer it only seemed like the ph was increasing, not decreasing. I use papers so not entirely accurate. I even use cacl in the milk this time as there was a question as to whether that would make the difference, but it made none.
When I use fresh milk I can make it, when I used store bought I don't seem to be able to do it. Very frustrating at the moment, and I wish I could answer your questions.
Ok just a couple of hints. Yes you need thick gloves, either a double lined pair or a gardening pair, they make all the difference. Also I put the water on to boil when I start the last stage of the mozz. That allows me plenty of time to bring the water to the boil. If I forget I use water boiled from the kettle to speed up the process.
Sorry I can't be of more help. I have looked and looked to see if I can find a "trouble shooting" video etc, but so far I haven't had any luck, and it is becoming an expensive trial and error campaign.