If one of your Pokémon loses an item to Knock Off, the item will still be gone until the end of the battle. Switching the Pokémon out of battle and then switching it back in will not bring the item back, only ending the battle will. In the game, when you check a Pokémon's status screen, it will still show that a Pokémon is holding its item, even if it has been knocked off of it.