DB Change Manager Schema Compare - target shows Unmapped with nothing in dropdown?

Trying to do a Schema Compare with DB Change Manager.  I've selected 4 schemas from the source for comparison - when I get to the Mapping page, the target database just shows 'Unmapped' for all 4 schemas, and the dropdown doesn't contain any entries.  I know my target database contains all 4 of these schemas.  Why aren't they being shown in the dropdown?  I've clicked both the 'Refresh Databases and Owners' and 'Restore Default Mappings' links, as well as checking and unchecking the Ignore Case / Ignore Spaces / Ignore Underscores / Show Users checkboxes, all to no avail.  How do I get the target schemas to show up and be selected?