Messages sometimes don't go through

My friend and I both have an iPod Touch 5th Generation. We often text each other and lately, it's gotten more common, the texts won't go through. Yesterday, it took 6 hours to finally go through. I've checked other discussion boards and I did everything in settings like I should. Here's the thing though, it's random. Also, my 4th generation iPod seems to always be the first to say there's a message. I have to open the application on my 5th generation and wait a minute before it finally updates. How can I fix this? I'm running on iOS 6.1 on both iPods, and so is my friend.