The reason there is no information as to why this is so, is that it isn't normal.
What avast version are you using 4.8 or 5.0 ?
So there is some conflict somewhere that is stopping this action.
What other security software do you have installed, firewall, anti-spyware, previous AV, etc. ?
It could also be that your installation isn't correct, so was this an update from 4.8 to 5.0 and if so did you do a clean install or install over 4.8 ?