Texting an non iphone user

My girlfriend has a Samsung S3 and I have an iPhone 5s for some reason she is no longer receiving my text messages yet i am receiving hers...whats the reason for this? i have seen this question asked before and not seen an answer. I have turned off an on reset network setting deleted and re-added the contact everything that the support site lists...does anybody have an answer for me please?

Thanks 🙂