readability doesn’t do any caching. plugins are simply not run, at all, if the article is considered up to date. it’s just skipped during update process.
whatever issue OP is having is not related to normal update process nor your feed contents. like @JustAMacUser posted above it could be his aggressive purging settings.
if (if-modified-since request header == timestamp of latest article) return http 304 (and don't generate any content)