Most Helpful Guy
Well, they are your ex. Even if you're still friends, you're probably not likely to be at the top of the list in the communications realm.
I don't think there's any special significance to not texting back, though. Some people are more comfortable talking than texting, or they could be driving, or any number of other reasons. I wouldn't attach too much significance to it. Have you ever asked them?
Most Helpful Girl
I am assuming that they would rather hear your sweet voice than to Continue pushing a pointless button of 'Text' that doesn't Talk.
Good luck. xx