So I've been playing on this world that I have, and I needed to find a fortress for blaze rods and such. I ran around for about 2 hours without finding anything, so I then turned on cheats and did /locate Fortress. It returned an error saying there was none nearby. I flew around the nether and continued the command and it always returned the same message. I then went on a superflat world and did the same command and it found one. As a final test, I started a new BOP world and went into the nether, ran the same command, and got the same message- "Could not find that structure nearby."