Is it possible that Plagg can't detect the other Miraculous or kwami because that part of his power has been sealed away? Unlike Tikki, Plagg also doesn't immediately recognise the book and doesn't have interest in it on looking inside it. At best, his memory of it is vague. The book in all likelihood has its own magic. Yet, Tikki recognises it immediately and knows that it's extremely important. Tikki also knows who Master Fu is and that Wayzz is his kwami, but Plagg doesn't notice Master Fu when near him. Since this problem seems to just exist with Plagg, that means the problem could be with Plagg's ability to detect and be detected.
Plagg has been near three Miraculouses, including his own in "Lady Wifi", and been unable to detect them. He's theoretically been near five of them total, and didn't notice that Lila's Miraculous was fake. The only one Plagg has definitely not been within a hundred yard radius of is the Bee Comb.
In "Stoneheart"/"Origins Part 2", at the end, he's close to the Turtle Miraculous and Wayzz, but doesn't notice. This might be due to distance between the two, since they are a couple yards away from Wayzz. In "Gamer" and "Evillustrator", he's as close as he can possibly get to the Ladybug Miraculous and says absolutely nothing later. Despite wanting to know who Ladybug is as much as Adrien and wanting Adrien to know who she is. In "Volpina", he's near a fake Fox Miraculous and doesn't notice, and is possibly near the Peacock one. He never tells Adrien that the Fox Necklace is a fake (Tikki, however, sees her buy it and knows that Lila is a liar) and doesn't seem to even SEE the Peacock Brooch despite being right near it. In "Simon Says", if Gabriel is Hawk Moth, he's close to the Moth Miraculous and doesn't seem to notice it, unless that sad look he wears is because he can feel that Hawk Moth is Adrien's dad. In "Lady Wifi", he's unable to detect where his own Miraculous went when it's knocked off Adrien's hand. It's therefore possible Plagg cannot detect other Miraculouses for some reason. Because Tikki never notices him, either, it's possible that Plagg can't be detected.
As to why his powers of detection were sealed, it might be because Hawk Moth is Gabriel Agreste, and it's to protect the Black Cat Ring from Hawk Moth. While it means finding Hawk Moth will be harder, it also means that it's safe. Thomas Astruc has said it was "strange" that Plagg didn't notice the Peacock Miraculous, so it seems unlikely that detecting other Miraculous is something only Wayzz and possibly Tikki can do. Tikki, as noted already, seems to be able to detect Wayzz, since she was able to direct Marinette to Master Fu for healing in "Princess Fragrance".
It's also wholly possible that Plagg was never able to detect the others, but then that raises the question of why Tikki and Wayzz can. Tikki, most likely, might have a self-imposed seal on her ability to be detected and to to detect at the very least Plagg to keep Cat Noir's identity secret.