I have the same problem on Galaxy s 4. It has been happening. It's intermittent but getting worse. I installed Verizon Messages in hopes it would eliminate the failed sent messages. I can be texting just fine then all of a sudden the a message to the same person will fail. I have to hit resend 3 or 4 times before it will finally send. VERY ANNOYING. I pay almost $400 a month to Verizon they need to fix this.
The failed messages happen at home, at work and other places. They happen with different people receiving the text. One time it was failing and failing. I removed the battery from the phone for about 5 minutes. When it finally rebooted the message failed again! Then the phone may go two weeks or more without a failed message.
My personal opinion is its a network problem.
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