Command block with the command /testfor doesn't work sometimes?

I was trying to create something when I tried to type a command with co-ordinate arguments, but it didn't work when the command block was below a certain level. Otherwise, it said that the player was not found, even when I stood on the exact co-ordinates. When it was below the certain Y axis, it said "Usage: /testfor <player> And the testfor command worked with co-ordinate arguments in another world.