Sky Replacement Not Highlighted

[dianab21793861]

Using latest version - editing old black & white photos - Sky Replacement works sometimes but with one the Sky Replacement feature is NOT highlighted. Tried saving it as .psd and/or .jpg and still not highlighted. Has worked with others in this group of B/W's.  

[S_Gans]

I have to imagine that with this image, the sky isn't clearly enough delineated, for PS to even understand it's there. Especially since the white sky matches the white background. 
Perhaps try "inventing" a sky, and temporarily crop the white edge out - so PS can see a "sky" there and know where to do it's thing...