I’m having the same problem in 5.0.507 as described here. Avast says the Shield is running, but regardless of my settings, including turning off all ‘Use caching’ options, no files ever get scanned. History shows 2 files were scanned 8 days ago, but that’s it. Unlike the original thread I referenced, rebooting does not fix the problem.
I started looking into this after I found an .exe in my Task Manager that, when manually scanned, was correctly identified as a threat by Avast. So I wondered how it got executed without being detected, and here we are.
Uninstall avast from Control Panel (if possible). If, for any reason, you can’t run it, try booting in Safe Mode and doing it from there. Anyway, boot after that.
Run the avast! Uninstall Utility saved on 1. If, for any reason, you can’t run it, try booting in Safe Mode and doing it from there. Anyway, boot after you’ve run it.
Install avast! using the setup saved on 2. Boot.
Register your free copy or add the license key for Pro.
Check and post the results.
Some infections prevent the correct installation of avast.
But, as Vlk said, difficult for us to say anything…