What do I do if a Sitter does not respond to an application for sitter service?

It's unusual to not receive a response from a sitter. In fact, sitters are penalized for not responding timely. Since you haven't received a response (or even if you do), it's advisable to reach out to at least a couple more sitters of interest and ask any necessary questions and set up a meet & greet to help find the right person.