Photoshop Elements :: Why Can't Edit Entire Photomerge Image

how to scan an old 1888,  10x12 photo  in two halves (left and right), and used the Elements Photomerge to combine the two shots.  It worked quite well - slightly askew at the top edge, but the rest of the image appears to be well-stitched - to the naked eye, at least.  
 
After sharpening the image, I went to Guided Edit and began using the brush to "heal" a few of the old marks on the image at full size - that worked well!.  Until  I moved my brush into the area that had been the left side, that is..  The brush no longer worked.  Then I noted that the about 1/3 of the left side was not sharpened like the right side was, either!    To test it further, I changed it to a black & white photo.   Only that larger right section changed to black & white, and the left part of the image stayed it's old sepia color.   I saved the images, closed and re-opened Elements - but had the same results. The left side just can't be edited.  
 
Isn't there a way to edit the entire merged image?
 
I've saved the image as a high-quality JPEG, and also as Adobe Elements format - but they still won't fully edit.    The two are large files - I was thinking that I'd need to save it at high-quality in order to end up with a nice retouched & sharpened 8x10, suitable for framing.
 
Also, I'm running Windows 7 Home Premium, 64 bit on one laptop, and Windows 7 Professional, 64 bit on another..(I'm trying to get by with the Elements version I have, due to finances.)