Sometimes weird behavior is corrected/cured by resetting the preferences.
1. Close Lightroom.
2. Hold down [Alt/Opt]+[Shift] while restarting Lightroom.
3. Overwrite the Preferences when prompted by the dialog.
4. Close Lightroom.
5. Restart Lightroom.
Does the behavior continue after resetting the preferences? If it does, are there any error messages?