Recents not updating with new files in High Sierra
When Recents is active in a Finder window, go to the window's action (gear) pop-up menu, select "Show Search Criteria". The first criterion for Recents requires that the "Last Opened" metadata entry have a value – this is probably why your screen captures are not showing up (if you were to open them in Preview, then they would show up in Recents).

You can edit the search criteria in that window, or perhaps better, copy the search string and create a new smart folder with it, then tweak it until satisfactory.