iPhone sends iMessage, but doesn't show that it was "delivered", even though it is
iMessage lets you send free text, photo, and video messages to other
  iOS and OS X users anywhere in the world . Read receipts allow your
  contacts to see whether or not you've viewed a message. While some
  people may consider read receipts to be a bit stalker-ish, others may
  find them useful for work and business situations. Regardless of why
  you need them, we can help you turn them on or off no matter what iOS
  device or Mac you're using.


http://www.imore.com/how-turn-and-read-receipts-imessage

Just ask to your friends if they have disabled this feature ;)