I to am having the same problem. I am beginning to believe it is being over cooked.
My cheese looks like melted plastic, very oily, and I have tried every cheese I can get my hands on.
I've tried Sorrento, Stella, GFS, all the brands that kroger, wall mart, giant eagle, etc, all turn out like plastic. I've tried whole mile grande and polly-o (couldnt for part skim for these two), plastic with more oil.
I have used same cheese as one of my favorite pizza shop (GFS), but mine doesnt look/taste anything like theirs.
I cook at 550 on a stone preheated for an hour, near the bottom rack.
Near the end of cooking my pizza's, I notice the cheese is intensely boiling.
I can almost see the oil being boiled out of it.
How long do I cook my pizza's? Well, I'm a newbie that has be concentrating on the crust, so I cook it until the crust is done. Now that I have my crust where I want it, I'm addressing the cheese problem. Yesterday, I tried putting the cheese in the freezer until near frozen, same thing, oily plastic.
My next pizza, I'm going to preheat at 550 for an hour, but when I put the pizza in, I'm going to turn the oven down to 450 or so.
Any other suggestions?