Hi kellykentAThotmail.com,
Well this is quite common for any resident av solution. It cannot catch all, to many variants, they have to make a selection for their database and cover the remainder with heuristics, too large a vulnerability window to cover and zero-days can also be your deal if you are so unlucky to stumble upon it online. This is not only avast it is with all av solutions, so best what you can do is have some additional non-resident scanning next to it (MBAM, SAS, online scanner of your choice) so the detection range become as broad as you can live with. Additionally if you want to be fully protected use a Mozilla browser with NoScript and RequestPolicy extensions installed, yep, and then, and then you are fully covered,
polonus