iMessage problems. (Unable to iMessage one particular person..?)

Hi randers4, thankyou for your reply but thing is she does have it on because she iMessages a friend daily and it works for them and same with me just not each other. When I have it on also it will say its delivered but she will not receive anything but her SMS still come through. If i didnt have wifi on it would fall back on SMS but seen as i do it says delivered yet she still doesnt receive it. And we have both updated to iOS 5.1. So we can't figure out who the one with the problem is. She says it is mine however I have a feeling it is hers but I don't know hence why I came here. Thanks Kind Regards.

I have tried to explan to her that it might be her phone but she won't accept it or want's to change anything as she doesn't want it to affect her phone if she does so I can not tell you if it worked or not is there anything else that you think might be causing it. She says its because of either the carrier we are on or the reason that she has iPhone 4 and I have iPhone 4S but still it doesnt explain the fact why she can iMessage another contact who also has an iPhone fine and vise versa yet not to each other. Very confusing. Thanks.

So what you're saying is EVERY iPhone that uses iMessage should have those setting not just the On/Off slide button for iMessage like she does? Thanks.

But how does that explain that she can message another iPhone user with just that setting? We've tried messaging same contacts and they both receive our messages e.g. I'll message friend one with iMessage on and they will receive it and reply and i receive their message then she will do the same and it works for them also but not for each other. Hmm mind boggling. Thanks.