I'm playing the Steam supported version of Half-Life and I've found myself at an impasse. I've reached the room where the Scientist tells you about the Crossbow. Further, I've successfully acquired it and killed the monster with it. The problem deals with the valve and the grate.

According to a number of walkthoughs I've read, turning the valve is supposed to raise the grate high enough to swim under. In my game, the valve turns; but, the grate doesn't seem to move. Am I doing something wrong? Is there a bug in the game?