Yawn will fail if the target has the Ability Insomnia or Vital Spirit or if it has Leaf Guard during harsh sunlight; a drowsy Pokémon that has Insomnia, Vital Spirit, or (during harsh sunlight) Leaf Guard will not fall asleep. Yawn will fail if used on a Pokémon without Soundproof during an uproar.

Easy fix - just switch out your Pokemon! Yawn makes a Pokemon fall asleep at the end of the next turn, so if you switch to another Pokemon, the sleep effect resets. Or, use a Berry that wakes your Pokemon up from sleep like a Chesto or Lum Berry.

In a Triple Battle, Yawn can only target a Pokémon that's adjacent to the user. Yawn will have no effect on Pokémon with Insomnia or Vital Spirit.

This terrain prevents affected Pokémon from being afflicted by non-volatile status conditions or Yawn. If an affected Pokémon is already drowsy due to Yawn, it is prevented from falling asleep. Rest will fail if used by an affected Pokémon.