If something about Lightroom is not working as it’s supposed to, resetting preferences can often resolve the issue. This video tutorial shows Lightroom Classic and Lightroom 6 users how to reset Lightroom’s preferences.
Users of earlier versions can also reset preferences (and should watch this video for explanations and tips), but the shortcut in the video won’t work. Instead you’ll have to go to Lightroom’s preference file using Windows Explorer (My Computer) or Mac Finder and delete or rename the file. (The advantage of renaming it is that if doing so doesn’t resolve your issue, you can restore its name and continue to use it.) This document indicates where the preference file (and other files) are stored for Lightroom 5.
Don’t want to watch a video? To reset preferences, hold down Alt/Opt-Shift as you start Lightroom, and you’ll get a dialog box to reset. This will set all Lightroom preferences settings back to their defaults, so you’ll then have to change any that you don’t want at the defaults. Taking a screenshot of each tab in Preferences before resetting can make this easier.
For best quality, after hitting Play click on the sprocket wheel in the bottom right and choose 720/HD.