Have a Canon FD/FL lens but it doesnt fit
Most likely the lens you have is a Canomatic or Caonflex lens. They are 'similar' to FD/FL mounts, but they do have their differences (https://www.cameraquest.com/canonflx.htm). Canonmatic lenses are also known as the Canon R mount lenses. Some of these lenses will fit the FD/FL camera mounts and some adapters, some wont.
Many of these lenses have a ramp or slope (see attached image) on them that interferes with the aperture pin on our adapters, I believe this is what you are running into. The pin on our adapter can be removed so the lens can fit, but you may not get aperture control then depending on the lens. On most of our adapters the pin unscrews from the outer ring of the adapter.