Apple is on top of iOS 7 issues, with the latest problem pertaining to Messages not being able to send entries from all users. The company has acknowledged the Messages issue in a statement to The Wall Street Journal, and mentioned that a fix for it is on the way.
“We are aware of an issue that affects a fraction of a percent of our iMessage users, and we will have a fix available in an upcoming software update,” Apple said in a statement. “In the meantime, we encourage any users having problems to reference our troubleshooting documents or contact AppleCare to help resolve their issue. We apologize for any inconvenience this causes impacted users.”
Messages has been having trouble since the release of iOS 7, and even during some of the betas that were issued to developers. I have had this problem on my iPhone 4S, though it was temporarily fixed when I restarted the iPhone. Doing so seems to solve the problem for a lot of users. If you not you can try this iOS 7 Messages bug workaround.
It’s possible that the rumored iOS 7.0.3 update Apple is working on will fix this issue and any others that have come to light since release. These types of bugs are expected with every new version of iOS, especially because this version is a major redesign