You might need to Run As Administrator once, if you do not have proper write access to a directory.
Another possibility is that you have something in startup.m or finish.m or pathdef.m that is overwriting the path so that it loses track of the package.
A third possibility is that your Add-Ons directory has been corrupted. If that happens, then you have to uninstall all of your add-ons, remove everything from the directory that MATLAB uses to store the add-ons, remove everything in the download cache, and then reinstall all of the add-ons. (This is the procedure Mathworks sets out. I would say that the situation cries out for some good package management tools!)
