This happens to me quite often when using the mobile app. (Only time messages just vanish on the website is when attempting to send to a free user that I hadn't noticed was a paid account, or an account that is EXTREMELY new).
When using the app, I've found that if I get sidetracked by the real world and allow the screen to shut off, then when going back and trying to send a message, it will ALWAYS just vanish. The only way to get around this is to manually close the app, and re-open it. But I'm sure this is all just our fault too "Don't let your phone sleep! SIMPLE!" hahaha

