The issue of high-gluten flour is an interesting one with pizza. I think it is a mistake to use it, at least use it completely. Gluten does two things to your dough, it makes it resistant to tearing (good), but also hard to stretch (bad). When I say "hard to stretch", I mean it actually is easy to stretch, but because of the elasticity of the dough, it tends to spring back into shape, just like a rubber band. Just two weekends ago I made pizza with 100% high-gluten flour, and had to resort to rolling it with a rollling pin. Tossing it in the air did nothing. A few years ago I made pizza with 100% all-purpose flour, and found it incredibly easy to stretch, but I had to be very careful because it was easy to tear. Usually, I do a 50% mixture of the two flours, and it works out fine, but I never get the kind of stretchable, strong dough I regularly see pizzarias use. Perhaps I am working the dough too much, or perhaps not enough. Putting more water in, as people have suggested above, is an excellent suggestion which I will try next time.