In order to understand why a communication queued for a certain customer in Core iQ using custom data fields that you had mapped in from outside of your regular fields, it is best to start with your own data sources. If the item was queued from an Actionable Report, then check your core data at the source for the time period at which you ran the report and queued the communication. If the item originated from an audience, check the custom SQL criteria at the bottom of the Audience Definition for any custom fields, then check your core data at the source to see if that custom data field was associated with the customer(s) in question at the time that the item queued in Core iQ.
Because we refresh all custom flags and data using the most recent versions we receive from your core files, it is easy for these items to be omitted or overwritten from one import to another if your core-level data changes. That is why we highly recommend starting your search using your own core data files which were imported to Core iQ. If the data files are correct at that time period, it helps us rule out certain issues that might have kept a communication from queueing in Core iQ.