I've been using Pr since CS5.5, and have learned that 90% of my problems can be fixed by resetting the Prefs. I suspect the Prefs are corrupted any time that Pr starts behaving unusually or unexpectedly: things stop working that were working before.
Here's my tip on how to cope:
1. Find the folder that contains the Pr Prefs, and make an alias to your desktop.
2. Delete the Prefs.
3. Launch Pr and set the prefs as you like them.
4. Quit Pr.
5. Open the folder you just located and zip the Prefs file.
You should only have to do this once for every version of Pr you have installed.
Now, every time you suspect the prefs are corrupt:
1. Quit Pr
2. Open the folder with your alias
3. Delete the Prefs
4. Unzip your clean version.
5. Launch Pr, and get back to work.
There's an app that manages Prefs in the Digital Rebellion Toolkit that will do this for you, but that's a paid app. And, you have to be sure to save clean uncorrupted prefs as a starting point. Be sure to do that right after you create a new set of prefs, and before you start editing anything.
AdChoices