We have seen this when there is a communication issue between the server and the device when using a Direct Queue.
The Print Service log will have more details on the reason why most likely.
Thanks for the advice, Jeff. I did a few traces in DebugView, matching times reported in the Transaction Log with events in the Print Service log and did not see any consistent errors. I see a few repeated instances of the following:
  [2017/04/26 15:05:13.743 P0C98 T0648 d PrintService 05148] [AI: 0527469e-e61c-46d8-b979-6a75de24a41b] I/O operation return with no success code. [errorCode: 64]
But those seem to happen a lot. Is this indicative of communication errors with print devices?