Synchronize folder does not fully import any longer

I typically use LR4 (currently 4.4) to scan a folder using "Synchronize Folder".  I have been doing this for months, and it worked as advertised, adding new photos and removing deleted ones.

However, lately after it does the scan, and the dialog correctly states the number of new photos, after the import completes only a fraction of the images have imported.  I need to repeat the process to get them all.

This makes no sense.

In other words:

-- I add 25 jpg files to the folder

-- I select the folder in my LR library and choose "synchronize folder"

-- the dialog says 25 new images, I click "synchronize" button

-- after the progress completes, there are only 7 photos added

Any ideas?