Why doesn't my imessage work?

It just says "sending" but loads up until the very end but stops and ssays not delived? And yes they are iphone/ipod users

Is best to try your support site or your owners manual for help, this site has owners manuals and service manuals, that will answer most question.
This site will help with problems, http://www.ccl-la.com/blog/
If this does not help try a web search, that will do the trick most all the time if you hang in there, it is all on the web.
This is the site for owners manuals
Good luck…