Hi,
That is a strange problem. Maybe the file you are trying to edit is write protected. Try saving to another place, with a different filename to be sure you don't get the files mixed up, and then open your new file and check whether your changes have been saved.
Another thought: could it be that you're trying to edit a file that you have added to an iTunes library? iTunes likes to copy files to its own internal folder, so if you edit the original then the one in iTunes will remain unchanged.
Alex