Known Existence: July 2002 (Publish 16, dates vary)
About 5% of the time, pets will not respond to the [guarded] flag on their owner. Even if the target that is attacking you is doing significant damage or something newly aggressed towards you, the pet still will not respond. Commanding the pet to guard you multiple times still will not make the pet attack your attacker.
Solution: cancel the original guard command via stop or attack, and then tell the pet to guard you again.