Why, why, why? Will the R6 not focus? I always get the red square when I focus on the eyes - and not the green. And yet when I switch to manual AF it's focusses fine.
Lens: EOS 70-200mm
Focal Length: 85mm
If this happens when using the strobe have you checked that focus assist is enabled for flas photography?
What kind of flash unit are you using?
If you look on P153 of the user manual, is says "the Speedlite will automatically fire a focus assist beam as needed, if autofocusing is difficult under low light". Thus this should work: if it doesn't check your Speedlite settings and if necessary refer to its documentation. If it is not a Canon Speedlite, definitely refer to the flash maker's documentation.
I'm using the Godox 400AD Pro. Have just done a quick search and there doesn't seem to be a focus assist beam with this model/brand?
You're right, I think it's a low light thing. Frustrating. I'm not sure why all of a sudden it won't auto focus when it has operated in similar set up, before.
If it DID work on this camera before, perhaps do a camera reset and see if some unknown setting has changed the situation?
save your current settings to one of C1-c3 modes, before you do a reset?