We’ve always seen one of two things in the ident field - a call sign or a tail number. Up until a few months ago, if the ident field was a call sign, we would also receive a reg field that would contain the tail number. Now we’re regularly seeing non-tail number ident fields with no reg fields, leaving us unable to determine the actual tail number. Any idea why that would be?
Some of our datasources do not include tail number information (for example, ADS-B does not transmit tail number information). You are perhaps seeing flights where there is no datasource for that flight that provides a tail number.
Beyond that general answer, we’d need a specific case to look at.