During compare and merge, A source physical DM does not allow target physical models to display as choices

I have 2 models, Model A and Model B.  These exist in separate models (separate DM1 files).  Both have their own logical and physical models.  I want to merge the physical model A (source) into physical model  B (target). 

1.  I select model A, while in the physical model, and select "compare and merge utility"

2.  In the next screen (step 2 of 5), and then select "compare against another model in a DM1 or DT file"  I choose Model B from the drop down, and click "Next"

3.. In this next step (step 3), it only displays the "Logical" model for the target (Model B) to choose from.  I.E.  No Physical Model displays to choose as the target.

So, I cannot merge into physical model B from physical model A.

However, If I start with a different source physical model, lets call that model C (in its own DM1 file, just lie model A), I can Merge and compare into the Physical Model for model B.  Model B's physical model displays in step 3.  That's what leads me to believe there is something wrong with Model A, and not Model B.

Any ideas?