Here are some suggestions on troubleshooting special characters and symbols in a PowerPoint presentation:

1. Check to make sure the text is saved in Unicode UTF-8 format.
2. Make sure the Font you are using supports special characters and symbols.
3. Check to make sure you are using an updated version of Microsoft PowerPoint.
4. Check the spelling of the text before inserting it into the Presentation.
5. Try inserting the text/symbol from a different source.