Why isn't Finder updating a folder's contents?
I've known it happen if memory was being paged heavily, or if a drive was particularly full. 

F5 has no effect on a Mac, btw, it's a Windows command.

The only real equivalent - to a system that really ought to never need to be refreshed under normal circumstances - is to switch view style…

…from list to column then back again, for instance…
 Cmd ⌘   3  then  Cmd ⌘   2