That happens with me and a friend of mine (I'm in Ohio, she's in Texas) from time to time and it's usually when she's in weak coverage or a heavy congested area.
I totally forgot to add that when I checked my Settings > About screen this morning for another unrelated issue, I got a popup notice that there was an update available to the carrier settings for AT&T, and did I want to update them.
It was doing the same with me yesterday.
I send a few imessages and they went through as a text message.
A few hours later I tried again and went through via blue bubble using imessage.
Weird....
I totally forgot to add that when I checked my Settings > About screen this morning for another unrelated issue, I got a popup notice that there was an update available to the carrier settings for AT&T, and did I want to update them.