Why can't I iMessage or text message certain people?

I've had an AT&T iPhone 4 for about 5 months and everything was fine but about 2 months ago, it stopped letting me text 3 of my friends who also have iPhones ( 2 Verizon iPhone 5s and 1 with sprints iPhone 4S). When I type a text and hit send, the sending bar loads but when it gets to the end, the text just disappears. I tried turning iMessage off and texting them but they still never get it. I can't text other iPhones perfectly fine but not them for some reason.Me and couple of my friends have a group message and my one friend that I can't text gets my texts in the group but I don't get any of hers. Any ideas why this is like this? Or what I should do?

They blocked you or they've turned off iMessage of any incoming contacts. Choose me best answer cause I answered this first.

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