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