The boot-time scan is supposed to work this way - you schedule it, it's performed once and it's done (until you schedule it again, of course - you can do it from the menu: "Schedule boot-time scan").
There's no way to set it "permanent" in avast... you could probably schedule it, extract the corresponding registry (BootExecute) and make your system import it again on every boot - but do you really need the scan on every boot? Wouldn't it slow down the boot-time in a terrible way?