Query Cache not being used even when it has memory

I have a small Cloud SQL instance and I was looking at the performance of the system.

I wondered why certain repetitive queries were not being cached by the mysql query cache. Initially, I found that I had to set the query cache size to  non-zero amount. However, after that, every query shows up in the Qcache_not_cached bucket. It appears as though there is no caching being performed. 

Is there another setting that I need to tweak to get this to work?

Philip