First this is the forum for the Server version of avast, so hopefully one of the moderators will move it to the correct forum, avast! 4.x Home/Pro.
The home version doesn't have scheduled scanning, so you are obviouldy trying some form of work around and I suspect you haven't set it up correctly.
Check out this work around out -
HOWTO: Scheduled Automatic Scans avast! Home Version!, there are a number of posts that people have had problems which have been answered and they may also answer your problem.
I can't tell you why, or even hazard a guess because you haven't told we what providers aren't running ?