I was unaware that that was how the two effects worked, so that makes perfect sense as to why when I cast it, it kept missing.

The real issue then is that I was able to continually cast Magic Barrier on myself without the recast timer being up. The base recast on it is 1 minute so I should not be able to cast it immediately after the spell animation completes. I used the recast command and checked my spell list to see that the recast time was working properly and in the spell list, the spell was grayed out as it should be. I was not casting from the spell list though, I was using a macro that I had set up for it. If Diamondhide was down, everything worked properly.