So, I caught a Shellos on Route 205 that knows the ability Storm Drain. After checking Pokemon Database, I saw that a Gastrodon with Storm Drain has an immunity to Water as well as Electric (I evolved my Shellos of course). I battled a Whiscash in the Pastoria City gym and Water Pulse was doing damage. I'm so confused about this since I never saw this happen before. Rental Gastrodon's with Storm Drain have the immunity, so why isn't mine working?