No other devices. Not sure why some would be delivered and others not based on email address setting.So just to pick some of the low-hanging fruit first...
Make sure her phone is set to receive iMessages using both her phone number and email address. This is in Settings->Messages->Send & Receive
Does she have multiple iMessage devices (iPad or Mac?) If so, check to make sure the messages weren't delivered to one of them instead of her phone.
Make sure when you send her a message, you choose her phone number (although this technically shouldn't matter if the above is set correctly.)