Issue with subdomain installation – can't create new site

I am running a subdomain multisite installation. My developer has successfully created two sites for me, plus the main site, and mapped them correctly, so I know that it’s doing what it should be doing. However, I cannot seem to create a fourth site using the “Add New Site” function. Here are the symptoms:

1. Dashboard for site 4 is created. However, it doesn’t not appear as an option from the “My sites” dropdown from the top menu bar. It does appear, however, when selecting Sites > All Sites.

2. Going to the domain x.domain.com doesn’t display anything. Just an empty page with no source code.

3. Blogs.dir folder for site 4 has not been created. Is this an issue? Does it get installed when a file is uploaded?

4. DB Tables *seem* to be installed correctly. In PHPMyAdmin, I see the 9 DB default tables that should be installed at the blog level.

Something funky is definitely happening. Would appreciate any suggestions you might have.

Thanks,
Christine