IMessage won't send to people with iPhones?

I was texting someone and it would just send as a text message and they have an iphone. I know their I message isn't deactivated either because my boyfriend texts the same person and it always sends as an iMessage. Any new contact I add it sends as texts instead of iMessages. My iMessage works too because iMessages send to my boyfriend. Does anyone know what is wrong with it? It's really irritating.

Go into settings > Messages > Turn of iMessage