Just because you can que specifically for a dungeon does not mean that it is in your random pool. The specific list is more lenient, allowing you to select dungeons that are a little higher or lower than your random pool. So it is entirely possible to not be able to que for a random despite being able to queue specifically, simply because the only dungeons are outside of the random pool. If you mouse over the die icon on the random que interface it will tell you what dungeons are in your random pool, if it's empty then you can't do it. (likewise, I don't think it'll let you que if there's only 1 eligible dungeon in your random pool, because 1 dungeon isn't very random). For more details : motion graphics videos