Thanks for posting.
It's possible that you don't have write privileges for the folder the file is stored in. Try moving it to the desktop and see if it allows modification there.
I had a similar issue and found this post in the process. The post helped me look closer at the file itself. I found that the file itself was locked on my mac computer so it would not let me modify it even though i technically had full access. I unlocked it and now it lets me have full access like normal. Hopefully this helps.