I finally figured out what was causing my problem and how to fix it and figured I'd post the solution here in case it might ever be helpful for someone else. The shortcut that wasn't working was the shift D combination to permanently delete a file or folder while in the explorer right click context menu. What had happened to cause this shortcut to stop working was an application I installed had, unbeknownst to me, added another menu item ahead of D
elete that also started with a "D," in this case "D
efragment." Once I removed that item (which I didn't need anyway) from the context menu, the shift D shortcut started working again.