Something that just came to mind, do you have avast set on silent/gaming mode ?
If so that can stop the virus definitions update.

Well you are getting the streaming updates, which is good (not totally out of date), are you getting any errors displayed when avast does its virus definitions auto update ?

If not, it could be time to try a clean reinstall on one of the problem systems and see if that resolves it.

  • Download the latest version of avast, 9.0.2016 (see below for download locations) and save it to your HDD, somewhere you can find it again (if you didn’t save your last download). Use that when you reinstall.

  • Download the avast! Uninstall Utility, avastclear.exe http://files.avast.com/iavs9x/avastclear.exe and save it to your HDD.

    1. Now uninstall avast! - using add remove programs, if you can’t do that (as appears to be the case) start from the next step, reboot.- 2. run the avast! Uninstall Utility, it will request it be run from safe mode (and will handle the boot into safe mode), once complete reboot into normal mode.- 3. install the latest version, reboot.
      Note: save the copy of avastclear.exe so it can be reused as some users are reporting that after running it, it is self-deleting.

Download locations - You can download the full 9.0.2016 installer and it will allow you to update.
Free AV: - http://files.avast.com/iavs9x/avast_free_antivirus_setup.exe
Pro AV: - http://files.avast.com/iavs9x/avast_pro_antivirus_setup.exe
AIS: - http://files.avast.com/iavs9x/avast_internet_security_setup.exe
Premier: - http://files.avast.com/iavs9x/avast_premier_antivirus_setup.exe