I've also found it possible for the screenshot folder to get full (for my computer at least, and I run on Windows) and that can prevent other screenshots from saving, so if you have any other screenshots in your screenshot folder already, perhaps try moving the screenshots to somewhere else on your computer out of the FH folder entirely and see if that works. But be sure to make sure your screenshot folder is in the right place also, as Enoki and Hakumi have mentioned.