Arceus: Arc Phone Tells Me "Seek Out All Pokémon" / Can't Catch Giratina

When I play the Azure Flute, my Arc Phone tells me “Seek out all Pokémon”.

I’ve beaten Dialga or Palkia (can’t remember which I picked, but I have both now anyway), found all the plates with Volo and then beaten Polo when he turned on me… I haven’t seen the credits, but as far as I am aware, I have beaten the main game.

The only Pokémon I am missing is Giratina and Arceus; I have at least one of every other Pokémon, as a “living Pokédex”.

Apparently I can’t catch Giratina until after beaten the main game and I’ve seen the credits; as noted above, I think I’ve beaten the main game, but I haven’t actually seen the credits (it never showed any credits after doing the above).

It says online that I need to give Cogita three pieces of wood to get Giratina - but when I try to talk to her, all she says is “I have passed on to you all there is to pass. Where history goes from here… I leave to you and your companions to see through.”

So right now, I’m basically stuck, unable to obtain the last two Pokémon.

Any ideas?