Nasus's Q did no damage to turret

I played Nasus last couple games. In my last game, i found my Q did about 250 dmg to turret when i farmed my Q over 650 stacks. After that game, i started a custom game to prove what happened. However, in this game, the dmg became regular which i did 900 dmg when i had similer build at 620 stacks. I wish someone could help me find out what happened to my last ranked game cause I could win easily if my Q dmg worked functional.