When battling in the battle subway with durant and slaking durant used entrainment on slaking and then it said "But it failed!". why is it doing this?
The moves Worry Seed, Entrainment and Simple Beam do not work on Pokemon with Truant.
GameFreak most likely made it that way so that the Slaking wouldn't be able to get another ability and thus be quite redicolusly overpowers. Gastro Acid can still be used for almost the same purpose, getting rid if Truant. Source: http://pokemondb.net/ability/truant
