Sorry for the delay. multi-site is supported, in fact I use it on my own production sites. There’s not much for me to go on here, but some suggestions:
An Imsanity network install has both a network settings page and a site settings page. The network settings can be used to prevent individual site owners from changing settings (ie, not allowing them to disable the plugin or change the max size larger than you want to allow on your servers)
I would first check whether the plugin is network activated (forced on all sites) or is available for individual sites to activate. Then make sure it was activated on the site in question.
Next I would check the max size settings to make sure the images actually do exceed the configured sized.
Finally Imanity will not always catch certain images uploaded via a plugin or a specialized them upload area. (ie, user content that a plugin controls does not alwasy go through the proper WordPress API calls and so Imsanity has no knowledge of the file being uploaded.
Aside from that, I’m not sure. The plugin was definitely designed with network install in mind so it should work.
For what it’s worth, working great on MultiSite for me.
