I searched the Equation/Symbols and did not find the "curly x" you mentioned. Could you hover over the symbol and specify the name?
Are you saving the file as a .doc file? .doc file format uses a different equation editor method and would turn the equation into an image if that were the case.
Another possibility is the type of font you are using may not be compatible. Which font are you using?
Please remember to click “Mark as Answer” on the post that helps you, and to click “Unmark as Answer” if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread.
Thanks!
Adrian Microsoft Online Community Support