[Possible Spoilers] A Certain Set of Keys

[PSFLeo]

Possible Spoilers:

Spoiler

I recently got the Klefki you can obtain after the chain of events starting with the engagement ring. I got super excited because I could run the AG set, Swagger + Foul Play, but for whatever reason it's a lot weaker than I remember. Are there any changes as to how Foul Play works? If so, could anyone tell me?

 

[Tartar]

There shouldn't be. 

Perhaps you're just using it against Weak Pokémon? You have to remember that not every foe you come across has 31 EV's, 252 IV's in Atk and an adamant nature. 

[Shad_]

In-game Klefki isn't that good, at least with that set

Swagger + Foul Play works way better competitively speaking (for the reasons mentioned by Tartar)