There can be a major issue with what is referred to as the "dreaded gum line". This is easily identified if you turn a slice over (top down) and use a razor blade or very sharp serrated knife t cut through the crust, then separate the cut slice and look at the cut side, if you see a well defined gray area immediately under the sauce, congratulations, you have a gum line. To further confirm the gum line take another slice, turn it upside down and beginning at the edge tear the slice apart in the middle as best you can, watch as the crust cleaves apart, if you see a web of dough being stretched (called feathering) between the two pieces as they are pulled apart this is confirmation of a gum line. There are about seven different issues that can cause the gum line (thoroughly discussed in one of my articles in PMQ Magazine) and until you identify the correct cause the gum line just won't go away, this is why it is called the dreaded gum line. Some causes: Dough stretched too thin; too much sauce; pre-saucing; baking at excessively high temperature; poor or incorrect dough management procedure or parameters; insufficient yeast level (generally correlates to poor dough management).
In any case the presence of a gum line results in a tough, very chewy eating characteristic.
Tom Lehmann/The Dough Doctor