Files in use or packed… Or too big… Maybe you can increase the Chest size into the Program settings.
Can you schedule a boot-time scanning?
Start avast! > Right click the skin > Schedule a boot-time scanning.
Select for scanning archives.
Boot.
If infected files are found, it’s safer to send them to Chest instead of deleting them.
This way you can further analysis them.
See also: http://www.digitalred.com/avast-boot-time.php