Recently, there has been an increase on holds not being trapped or “getting stuck” on certain call numbers.  

This seems to be related to book call numbers having the |z analytic.     

The |z analytic must be used in call numbers only for records with multi volume items and serials records, in which all the different volumes are in the same record. 

See the error in the example below: this is for book 3 in the Bones series.  This means all the items in this record are for book 3; therefore, the |z is NOT necessary.  

Incorrect call number: 

The bibliographic record has the information that the record is for book 3, but if a library really thinks it necessary to add the book number to the call number, you can do so, but without the |z.

For more information:  When not to use volume information (|z analytic).