Android App Install Problem

I have a generic android phone / tablet (7") which has been working fine then the other day I tried to install an app and was told that there was 'insufficient space'. I cleared out the caches of all the apps and ensured that was could be moved to the SD card was moved there.

It would appear that apps are not being stored on the SD card when installed. I now have 6.67GB available on the SD card and only 18.6MB left for internal storage. Is there any way I can force the unit to install apps on the SD card instead of the phone?