You might want to check what value you have set for Minimum Free Space for the shares in question? It is not obvious, but the larger of the global share setting and the individual share setting is used to decide if the file should go to the cache. It is also a good idea to use the suffixes rather than entering just a numeric value (especially as that value is not an absolute number but the number of KB) as it can be easy to get the number of zeroes wrong.
There has been a feature request raised that the User Share value for Minimum Free space should not apply to the cache, and that the cache should only use the value set under Settings >> Global Share Settings, but I have no idea if that is going to happen.