Theme Magic is a powerful tool for making non destructive, real time changes to the style of your Joomla template via the T3 template's administration interface.
Admintools has a feature that allows the tightening of behaviour on specific file types on your server via creating a custom htaccess file. This is an incredibly powerful tool for improving the security and performance of your Joomla site.
Theme Magic uses LESS files to make the real time changes to the design however if you are using Admintools then you need to make an exception to these files in the admintools interface.
If you do not make an exception for less files in the admintools configuration then Theme Magic will not work as expected.
In the Admintools htaccess generator configuration area you need to add the less file type to the list of allowed files as per the screenshot below:
The default settings of admintools professional web application firewall will prevent the loading of certain files needed by T3. This can result in some less and javascript files not loading in thememagic and also no menu items showing in the megamenu edit screen.
To fix: Go to Components > Admin Tools > Web Application Firewall > Configure WAF > Visual Fingerprinting Protection and set "Allow Site Templates" to "Yes".
Another common reason for why the Theme Magic interface will not load is the presence of a javascript conflict on the front end of the site. Since Theme Magic renders the preview of the front end of the site any errors on the front end will also affect the behaviour of the Theme Magic functionality. Theme Magic uses javascript to manipulate the results of the Theme Magic preview.
Our tutorial on how to resolve jQuery errors is a good starting point for determining if you have a javascript error that may be preventing Theme Magic form loading.