Flour, especially higher protein flours (around 12% protein is good). The flour in the sourdough starter is fully hydrated, which can weigh down the dough and make it stodgy (I love that term. It means heavy, borrowed from The Great British Baking Show). You need that extra protein so the pizza dough maintains its gluten structure since it doesn't have a second rise after shaping. Pizza flours (aka Type 00 flours) range from 9 to 15% protein content, so you'll want to be mindful if you choose one of those. The King Arthur Baking All-Purpose Flour has a protein content of 11.7% which works well (that's what I use with good results). Also, since sourdough pizza dough has a 60% hydration level, the dough will be stiffer than a regular pizza dough. Flours with too high a protein level will be "thirsty" (that is, absorb more water), and will stiffen the dough even more. You want to strike a good balance so the dough will be workable.