Several years back, an incoming human caravan lost a wagon before all the caravan units were on screen. The caravan units on-site turned around and left, then multiple future caravans from the same entity failed to appear at all. I eventually loaded the save in DFHack and confirmed that there were still human caravan units loitering in the unit list. Dismissing those units using DFHack allowed new caravans to arrive successfully.

This is a seasonal autosave in summer 262, immediately prior to the non-arrival of the human caravan that motivated me to load the save in DFHack and resolve the problem.

http://www.bay12games.com/dwarves/mantisbt/view.php?id=9593