Tiny Thumbnails

Ver 3.1, Center Row theme (though this problem was happening on every other theme too)
For some reason recently all the new items I’ve uploaded have generated these tiny 50x50 thumbnails instead of 800x800 ones like the rest (and I think those are too large anyways).
Even on the backend the file preview is tiny.
I’ve tried changing the thumbnail settings but they do not seem to affect these.
Is there a way to forcibly regenerate the thumbnails?