Did you try trashing preferences?
Did you try to export the file to IDML and re-open that IDML file?
Did you do any font corruption tests?
Thank you for your response. I am sorry but I don't know how to do 'trash preferences' or how to do a font corruption test. It has happened in more than one font. I did try and export it as a IDML file but that was after the problem happened.
Any more help greatly appreciated.
Can you tell us more about the changing case, please? Is it all lower case i's, or only those combined with certain other characters? Does it happen in all files, or only specific files?
Have you checked for a possible auto-correct entry that might be doing this, or a GREP style in one or more of your paragraph styles?
Is it always when next to the letter F ? If so, it might be the dotless i you are seeing as an automatic ligature.