books remain in processing queue

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Books remain in the 'Processing' queue of the republshing dashboard and do not advance to the 'Republishing' queue.

Here is an example:
- http://scribe10.shenzhen.archive.org/0/items/process/sixteenfamouseur00cerf/

note: this issue also occurs with the following books:
- http://scribe2.shenzhen.archive.org/0/items/process/abzsofcooking03wing/
- http://scribe2.shenzhen.archive.org/0/items/process/lostfound00siga/

note2: these items have been reprocessed multiple times with the same result

note3: all 'raw_jpg' images are present, but there are not any 'orig_jps2' or '_jpgs' images

note4: the 'ps_log' text file is empty