I have an iMac running OS X Yosemite (version 10.10) and Avast Mac Security 2015 version 10.0 (43029).
I have noticed that it no longer automatically updates the Avast definitions or programme.
I am not sure if this is since Avast updated to its current version, or if it is since I updated my Mac to Yosemite, but it is a fairly recent issue.
It is not the case that there simply had not been any updates - there were, it just did not download them automatically. If I have had no updates for a few days, and open Avast so that I can manually update it, I can see from the update log that there have been daily updates in between that I have not received.
I have also noticed that manually updating requires entering my Mac’s password every time, into a window prompt that always says “Avast is trying to install a new helper tool…”.
I have the same problem. It does not update even thought the automatic update is checked. If you need another support package download let me know. Always glad to help.