Can't access admin-ajax.php on some sites on Multisite install

I have a multisite install on wpengine, with the main site acting as the parent theme and each subsite a child theme.

From the main site and one sub site I can call /wp-admin/admin-ajax.php without any issues from users with a subscription roll. On the two newer sites (since 4.0+ came out) I keep getting the generic error "You do not have sufficient permissions to access this page."

Now, I've google-fu'd ad nauseum on this topic and here's what I've tried:

• Disabling all plugins and switching to a default theme


• Disabling/enabling parent and child themes


• Verified network site settings (identical)


• Verified db options, correct prefix etc

Notes:

• Both subsites were copied using wpengine clone feature

I can register a test user to the site without any issues, that test user can browse the frontend without trouble. According to the wp codex and source code, a user simply needs to be logged in for wp_ajax_{hook} to fire; however, the code doesn't seem to be executing that far.

My gut is telling me that this is a configuration problem somewhere. Any insight would be helpful.

Thanks.