Let's start with these questions:
Is your yeast dead?
Are you overbaking your pizza?
Is your oven too hot?
Did you let your dough rise long enough?
Did you use enough dough?
Is there enough hydration in your dough?
Did you overknead it?
Did you use too much oil?
Did you use high gluten or bread flour instead of all-purpose?
Any of the above could be the cause, or...
There could be another thing you're doing that we'd need to investigate:
Could you give more details about your last attempt?
What are your expectations for the crust (thick/thin/flaky/crispy/doughy,puffy,springy/chewy?),
what size pan (and type) you bake with,
what was the last recipe you tried?
what ingredients did you use and how much?
what methods did you use for mixing/kneading dough, rising, baking, etc?