Why isn't Finder updating a folder's contents?

Today I had the strange behavior that I added some files to a folder in Finder (that happens to be in dropbox) from Outlook, but Finder did not show them. I could see them from the Open File dialog of Outlook and of Adobe Acrobat Pro XI, but returning to Finder, still not there. Hitting the F5 key to force a refresh, still nothing. Finally, I forced Finder to quit and relaunch, and they showed up. This is OS X Yosemite v10.10.2. What is going on? Why would Finder not show the new files until forced to restart?