Decoupler block not responding to key presses

I was experimenting with rockets today, and decided to try to make a multistage rocket using the altimeter and four decoupler blocks to eject the previous stage. When I tested it, though, the decouplers didn't fire in response to the altimeter, or to a manual key press. Even when I connected two blocks with a decoupler separate from the rocket just to test the block, it still didn't work. Has something broken this mechanic?