Safari Tabs Won't Move To Another Safari Window

I have been dragging my safari tabs, as I usually do, to my other safari windows. This time they refuse to transition over to the new window of the same browser. I can drag it, and once I let go of the tab, it disappears and persists on the original window. I've tried messing around in the Safari > Preferences > Tabs settings, and nothing seems to change. How should I troubleshoot?