iMessages won't say "Delivered" but are still sending

Starting yesterday, my iMessages no longer consistently say "Delivered" when they have been sent and received. Sometimes they do, and sometimes they don't. However, I have noticed that there isn't any actual problem with the message transmission -- even when there is no "Delivered" text, the message has been delivered. Has anyone noticed this issue, and is it really just the fact that the word is now missing? I just want to be sure there are no other problems at hand.