What could cause my gas oven not to ignite?
Since the oven has two burners, it would of course have two igniters - but either igniter could be busted and the other would still make noise. Can you look at the igniters? Are you sure both are firing? Because it would be easy for something to have fouled the lower igniter. 

You really want to see that the igniter sparks at least as bright as the upper one before you say that it works.