I get a "message not sent" error for most texts I send but people still get them. Is it my phone, or does Sprint suck baboons a$$?? Hah sorry, so damn annoying! I'm using stock. Tried chomp, thought it would be different, but same mumbo.
Had the same issue on mine. It wasn't all of them, just a few. Usually happened when I was sending several texts to different contacts in a short period of time. Was worse in bad signal areas. How is your signal when you have this issue?
I also want to add that just before this started happening, messages would be stuck "sending". Also, some people's messages will come, it'll ask me to download and it never will. They end up saying "0kb null. " So frustrating!
WiFi has nothing to do with it. It all boils down to your cell's data connection. Even in mediocre connection areas, your text may get through but your carrier does not receive a confirmation that it got through. You will either get an error eventually or the text will get resent several times until the confirmation comes through.