I was playing Omega Ruby over the week and my Gardevoir was paralyzed by Gloom's use of Stun Spore. Then, the wild Gloom used Sleep Powder and it told me that the move failed. Is this because my Gardevoir was already afflicted with Paralysis or because of some other factor?
A Pokémon can only be afflicted by one non-volatile status condition at a time.
A non-volatile status condition is a status condition that remains outside of battle and after being switched out.
These are: Burn, Freeze, Paralysis, Sleep and Poison.
A Pokémon can be affected by multiple volatile status conditions at a time.
A volatile status is a status condition that will wear off when a Pokémon is switched out of battle or when a battle is over.
These are: Confusion, Curse, Flinch, Infatuation, Seeding, Taunt...