Thread:Project:Support desk/wgUseInstantCommons doesn't seem to cache images

I have enabled wgUseInstantCommons on the wiki I develop (http://practicalplants.org/wiki) but it doesn't appear to cache images, and appear to be requested from the commons each time they are displayed. If I load a page which pulls in images from a number of pages, it's clear from the long wait that they are being requested from the commons each time (eg. http://practicalplants.org/wiki/Search). Also, if I disable commons then none of the images previously loaded are available - I'm assuming that would not be the case if caching was working.

I have tried replacing $wgInstantCommons=true with the $wgForeignFileRepos[] value defined on http://www.mediawiki.org/wiki/Manual:$wgUseInstantCommons#Details to no effect.

Is there something I can do to debug this? Are there folder permissions that need to be checked?

Thanks! Andru