Queue callback calls stay as pending

Customer reports queue call backs ( vqplus_callback) calls stay as pending unexpectedly. 

Initially customer's callbacks stopped completely, after restarting the service, all pending calls were being done without considering queues agent capacity, resulting in overcalling

Customer stopped service, and had 211 calls in pending, Bryan Walters flushed vqplus_callback_calls table, then service was restarted and operations began again. 

Later, another set of calls stayed as pending, again, table was flushed. 

Now (as of 1-jan-2018) there are calls pending when executing  select * from vqplus_callback_calls; and in the callback report for queue 9000. 

There is no apparent correlation between what makes the calls stay as pending or what caused the 211 calls to stay that way either.