1. Make sure you have the correct font installed on your computer.
2. Double-check that you have used the correct font weight when you italicized the text.
3. Check if you have any display settings that might be overriding your font settings, or interfering with display of the correct font.
4. Make sure that the correct font weight is still set after you have saved and reopened your PowerPoint presentation with the mediated font settings.
5. Try to reset the font to its default settings.