Piorun's reserve role against Bismarck

Why did ORP Piorun not fire torpedoes at Bismarck?

Piorun had not yet trained for the coordinated night box attack, so she remained ready to reinforce a threatened corner. Officially she did not fire, though that detail is treated as uncertain.

Piorun had not yet trained for the Royal Navy's coordinated night box attack. She remained available to reinforce whichever corner Bismarck tried to escape through. Piorun and the destroyer at that corner could then use their torpedoes to force Bismarck back or sink her. Officially she did not fire, although the possibility that she did remains open.