1. Some plugins or themes switch our cache function off (often for your whole site) without this being reported. You can find this out by looking at the response header; if the following line appears (or something very similar), the cache has been switched off.
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
You can see the response headers via developer tools in Chrome (F12) under the ‘headers’ tab. In FireFox you go to the 'network' tab; you can see the headers of the request on the html page itself.
2. Our caching will not work if your website uses sessions. If one of your plugins, or your theme sets a PHPSESS cookie, Varnish will be skipped and requests will directly be passed to the backend