Not receiving Text Messages on my Iphone 4 from husband's 4S

I have an Iphone 4S. My wife has an Iphone 4. About four weeks ago she started to only receive my text messages sporadically. In the last few days she has stopped receiving them altogether. She receives messages from other people, just not from me? This problem started happening before the new IOS. How can I get her phone to start receiving my messages promptly (or at all)?

andy