Caching not working on page with asynchronously loaded asset transforms
Seems to be the case that Craft won't attempt to resolve these /cpresources/transforms/ URLs until the asset is actually requested, which can mess with a setup like PictureFill where there is no guarantee that the transformed assets will ever be loaded.

Solution in my case was to to set 'generateTransformsBeforePageLoad' =&gt; true, in your Craft config array as mentioned here

After doing this, all transform URLs resolve and the page is properly cached.