Clone source problem

Routinely, I clone from one document to another in PS (current CC version).  There is some setting or "aspect" of my current document that prevents me from cloning into it.  I can't figure out what it might be.  I can create a new document and clone into that.  And it's easy enough to figure out a work-around.  But, given that there are only a few variables, it's driving me nuts that I can't figure out what the problem is.

The following pic shows the doc that I'm trying to clone into (L) and the doc that I'm trying to clone from (R). Also, the two simple layer stacks. The clone source palette correctly identifies the source file (moss.psd).  I select the source file; click+alt to sample; select the destination file; move the clone stamp across the surface...and nothing. I see the source image in the clone stamp circle, but no image is transferred.  Any idea what the problem might be?