We're currently aware of an issue where some font types are not supported in the writer when you try to use italics. We are currently working on getting this fixed.
In the meantime, please open up any story as a reader and tap your screen. An (Aa) icon should appear in the top right. From there, choose the Times New Roman or Georgia font setting. After doing this, you can go back into the writer and use the italics feature as normal.
Thanks for your patience and understanding!