Why doesn't iMessage send my pictures?

Recently, iMessage hasn't been sending any of my photos, GIFs, and videos. Especially from my iPad. They say that they send but then afterwards it says "Not Delivered!" That has happened a number of times but normally the other person will see it even when it says it's not delivered. This time, they say they do not receive it. This also happened to a friend of mine on her iPad but she said that it just fixed itself one day and she didn't do anything to it. I want to fix this because it's starting to get really annoying! I can receive GIFs, photos, and videos but I can't send them.