Why are my texts so delayed with one person?

I have an iPhone 4 with tmobile and she has an iPhone 4s with sprint. And the texts are like an hour late or I just don't get them at all. I don't know how to fix the problem we've both restarted our phones. My conversations with other people are working just fine but between me and her it's not working.

I think she has problem in her network thats why you both are facing problems, i suggest you to asker her to change network