You'll need to open the project that you are having trouble with and then open the original family definition from disk. Do not simply select the family definition in your project and click "Edit Family" as you'll be editing a corrupt version of the family.
Once you have the family open you'll need to click on "Family Types" from the design pane. Select the "Width 2" parameter and change the value from 12" to 14" and click Apply. Now change the value back to 12" and click OK (yes, I know this doesn't make sense, but just go with it). Now click "Load into Project" on the design pane. When prompted click Yes, to reload the family.
This should correct the family definition in your project. If it doesn't please post back and we'll see if we can figure out what the problem is. You also always have the option of contacting support and having them help you walk through the problem.
jason