Firming up Neapolitan Pizza Crust
500F is not really that hot, in terms of Neapolitan pizza.  Traditionally, they are baked for 2 minutes or less, at 700F - 900F (371 - 482C).  So, this could be a temperature, and/or a cooking surface issue.  What type of oven are you using?  On what type of surface is your pizza cooking?