I noticed like others have that my original Bose model QC35 headphones would not pair. I beat on the switch all I want, but no ready to pair voice.
I took it apart following youtube instructions and did the rubbing alcohol fix. That seemed to work while the headphones were apart, but once I put the back plate back on, no go.
So now, I take the back plate on and off and the consistent problem is that it will pair all day with the cover plate off, but never while the cover was on. Eureka! We have a switch throw problem.
So I took out my drill and a sharp bit and proceeded to mangle the cover to test this theory. Note that I didn't do a good job following the angle of the switch, but it is functional. You could do a much better job, but faced with the death of a nice older set of otherwise functioning headphones, I didn't take much care and mangled away. A good thing to do from here would be to use a black sharpie to color in the edge so that it is not as noticeable. I hope this gets out there and helps somebody with the same issue.
Update June 10, 2020: This fix worked for me for a bit, but the switch was rotating inside the headphone assembly. I decided to remove the plastic power switch that is normally used entirely. This leaves a tiny power switch that is not handy to get to inside the headphone. I am now accessing it through the enlarged hole and using a pen to reach it and power the headphones on and pair bluetooth that way. It is not ideal, but it is working fine for me.