Generally speaking, "unable to scan a file" result is due to that particular file being an archived file and also password protected. Since Avast does not know the password required to open that file to scan it, it reports it is "unable to scan a file" in that instance.
You could put the increased number of "unable to scan" results down to an increased number of similar files you've accumulated over time.
Is an archived file malicious? No, cannot be, at least not until it is opened to, say, run the installation file for a program.
If it [is] not opened, it is inert and presents no danger to your system (even if there is malware inside).
