Some possible scenarios for example:
1) You have Avast UI opened and select some file/folder to be scanned from shell extension menu
2) You have Avast UI closed and select some file/folder to be scanned from shell extension menu
You can try the same file/folder once/more times, try different files/folders for each attempt.
Is the progress of the scan displayed every time? Are there results of the scan available? Is the scan even finished for each file/folder?
Etc.
OK Jan, tested it as suggested above...
Result: It sometimes works and sometimes it won't.
Couldn't find any obvious pattern, sorry.