Recently imported photos stuck in Imports

I am a longtime Mac user and I am constantly importing pictures to Photos from my computer. For some reason, today when I imported several new pictures from Dropbox (something I do nearly every day), they remained in Imports and will not show up in the Photos Library. I checked my Photos preferences, even though I knew I hadn't made any changes, and the photos are supposed to repopulate from Imports to the library, as they have always done in the past. I also tried quitting the Photos app and opening it again, but with no luck. Since the pictures won't go into the Photos library, they won't copy onto my phone or iPad. I'd love some advice! (My new iMac is less than a year old, by the way.) I can't imagine why something I have been doing every day would suddenly, today, not work. Thanks!!