Why isn't my mozzarella stretchy?

I would agree with over handling as a problem.

When I hosted a mozzarella party, we initially had the same problem. We decided that over-handling the curds made them hard to stretch. It is fun to knead the curds, so people sometimes go overboard. When we started applying a quick, more gentle touch, we found we could stretch the mozz over our heads!! (And yes, it still tastes good, regardless!)

Alas we'd planned on it for dinner. I think we're just going to eat it. Kids think it tastes fine, but it's a bit tough. Thanks. Maybe I'll try another time without kids' assistance, perhaps that messed us up.

If you want to keep trying, I say stretch a little, then dip back into the hot water to keep it pliable. You could save the curds in the fridge for a day and give it another go. I've done that when I've felt like my curds were just too delicate and crumbly to stretch properly and it's turned out fine.

Yeah, it was really hot! And the curds looked all soft and mushy, like in your photo. But then I could only pull maybe 1/4" before it broke.

Is your water hot enough? You want the curds to be really melty before you get to stretching and pulling. It may be too hot for your bare hands, so food-safe gloves are a good idea.