Channel Mixer acts differently, depending on how you got B&W.
If you try different ways to convert to B&W, you’ll find out, in which cases channel Mixer will work.
One of my favourite ways to B&W is to use the HSL tool. It can be used to change tonality of each channel AND each channel can be tailored too by making it wider or narrower or moving edges. Here’s a video with some OTT changes:
Screen @ 720 @ 2024-11-08 um 16.27.40.mov.zip (45.5 MB)
At the end of the video, I re-add some colour. This can be used for creative purposes like a B&W image of a poppy field - and only the petals are coloured. The example used is a lot more boring than that, alas!