BDSP Problem with Swarms

I've been doing a poc on my copy of Brilliant Diamond for quite some time at this point I've finished everything I had to do but I'm still missing 3 Pokémon i can only get in swarms. Is there something I've been doing wrong or just bar luck? I've been checking my game (almost) every day for almost a year now and i still don't have them I'm starting to wonder if it's a bug or something. Has anyone experienced something similar?