From the Model Mayhem FAQ:

Why isn’t anyone viewing or responding to my casting call?
Unfortunately, we cannot say exactly why some people may or may not respond to casting calls.

Model Mayhem is set up to give our members full control over their careers as artists. This means that they can choose to accept work or ignore requests for work if they so desire.

If no one has responded to your casting call, it could be for a number of reasons. The job, pay, or even the wording of the casting might not be attractive enough to members for them to inquire further. They may not be available the days of the project.

Here are a few tips to ensure your casting is the most effective from our Edu article on Recruiting Talent through Sponsored Casting Calls.
Finally, if all else fails, you may try sending out some Proactive Alerts.