Since you don’t mention your operating system or scheduled scan settings, it is hard to hazard a guess as to why ?

There is a setting in the scheduled scan not to scan if running on batteries.

The other issue on OS, win8 by all accounts does a strange sort of shutdown (fast shutdown) which is kind of like hibernate and I don’t know if A) that is what happens when the laptop is closed or B) the hibernate is something different to the usual sleep, which avast is meant to be able to wake up to do the scan.