In Pokemon Red, I have a Pidgeot and it knows the move whirlwind. But whenever I use it in a trainer battle, it doesn't affect the opponent's Pokemon! Which Pokemon species/types are affected from this move?
Whirlwind has a different effect in trainer battles vs. wild battles. Against trainers in the first generation game, it has no effect. Starting in generation 2 trainer battles, it forces them to switch out to a new pokemon at random. If they have no other pokemon, it has no effect.
No pokemon typing will cause immunity, but the ability Suction Cup or the move Ingrain will cause it to fail.