Dave, thanks for the reply. No matter what I do, flatten, merge, select all 3 layers the result is still the same. I'm convinced if you have an image layer above another it must have reduced opacity or it will 'block' anything under it. I have all 3 layers active (the eyes on the left) the middle one selected in blue is just the layer that is active for adjustment. But I have tried or suggestion but still no joy. Thank you for reply though the thread being a bit old I did not expect a reply! The result in 3 layers has made such a difference to my North American Nebula image it is worth trying to find a way.
Cheers Hugh