Comment 1 for bug 1430545

The table action.unfulfilled_hold_list keeps a record of holds that should have been filled by a current targeted copy but which were not filled. It is populated each time the hold targeter processes a hold for retargeting.

The hold targeter also consults the "Maximum library target attempts" library setting. If items at a given library have failed to pulled the number of times specified by that setting, the targeter will try exclude that library's copies from future consideration.

Does that give you enough to work with?