w/ one person in particular that i text w/ often, our text convo's are being sent 50/50 between imessage and regular text. We both have full service yet its still happening sometimes it allows me to send as iMessage, but then it will take 2-3 minutes to "deliver". I'll lock my phone and then 2 minutes later here the sent sound. I'll look at the text I sent to see that it went through as a text instead of an imessage. Any idea why this happens?