Siri not recognizing nicknames

Since I upgraded to iOS7 Siri doesn't seem to recognize nicknames for contacts I have. For example, I have my sister's contact saved as her full name with her first name as "Elisabeth." I have her nicknamed as "Liz," because that is what I call her. I frequently say, "Text Liz [message]." Siri will then ask, "Did you mean Elisabeth?" I say yes and then Siri will ask me to confirm. Often times this extra step even deletes my message and I will have to say the content of it again. After I send that message, Siri will apologize for not knowing how to pronounce the name and ask me to pronounce it, except Siri is expecting me to pronounce "Elisabeth" and not "Liz." If I say "Elisabeth" Siri accepts my input but nothing changes. If I pronounce "Liz" Siri doesn't recognize my input and after a few failed attempts will suggest I put a nickname on her contact-WHICH I ALREADY HAVE. If I delete the nickname and then re-update it, Siri might recognize the name once but then it goes back to not working.

Does anyone know how to fix this?