There was a server reset on the server that I'm on, and we were allowed to take things to the new world. I took my Iron Tank because it's a lot of iron. Before the reset, it was working fine. Now, I've set it up and it didnt work, so someone on the server suggested that it might not be the minimum size. I made it 1 block taller and it still wouldn't work. I also filled the inside with dirt and removed the dirt in case some block other than air was inside the tank. Still nothing. It's a 5x5x6 iron tank (unless I'm measuring incorrectly, then it's a 3x3x4). Also, this is on the Unleashed pack (1.1.2). The only change made to the server before and after the reset was enabling Biomes o' Plenty. The tank is in a floating island in a Highland biome nearby a Gardens biome, if that means anything.
http://imgur.com/a/RNb7J
http://imgur.com/a/RNb7J