Messages gone

When I try to send an iMessage to a person, it automatically sends it as a normal text if the other person doesn't have internet at that moment. Well, the problem is if it does that, it doesn't send the text, I lose all my messages, and I have to reboot my iPhone device, turn my wifi off and send it as a normal text. Can anyone help me?

Note: I also had this problem before the latest update.

Greetz

Emile