According to several user reports, it seems that after updating to version 4.0, these users have encountered problems sending MMS. Basically whenever they try sending a message, they will be greeted by an error message that says, “Message not sent. Touch to retry.” Repeated attempts and multiple messages don’t seem to have solved it either.
There is a thread on the Hangouts Help Forum where many users are reporting this issue themselves. Thankfully it looks like Google is aware of the issue but it is unclear when a potential fix will be released. In the meantime, users have suggested a variety of workarounds that may or may not work for you.
Some of the workarounds involve downgrading to version 3.3. Others include checking your APN settings. Another has suggested clearing your app’s data, and another involves changing your settings so that SMS won’t appear in your Google account. Like we said some of these workarounds might work for you as users have reported varying successes with them, but feel free to try it out all the same if you’re experiencing these issues.