I had this problem with my sister living in Austria and I am in the States and at first I wondered if it was some international problem.
Are you sure you both have your apple ID's programmed into your contacts? For some reason it wasn't registering my sister's phone number as an iPhone but when I added her apple ID e-mail, it sends fine. I just have to iMessage her to THAT particular e-mail. That's the only way I was able to sort this same problem. iMessaging her to that e-mail works just fine.
I've iMessaged people in the states before just using their phone number only and not having their apple ID e-mail but with messaging to my sister in Austria, I have to use her e-mail. Wondering if anyone else has this issue with international iMessaging? I've iMessaged someone in Australia before with just their phone number and it worked fine so maybe it's completely random? I don't know. But definitely check and see if you have their apple ID in there in their contact information and try to iMessage them on that. See if it works! It did for me.