Anyone working in LE who has ever had to try and trace a crime gun or other firearm when it comes back "record not on file" has had occasion to find out how many dead ends can be encountered, and how it's not nearly so easy to trace firearms as TV/Movies makes it seem. Especially older guns. Guns made before serial numbers were required. Guns that made it into the country without being imported. Guns that changed hands in both legal and illegal transactions without records being made.
Then, there's the always entertaining instances where the original records (DROS) contained mistakes that were never caught and corrected. And never can be, due to businesses being closed, records keepers being deceased, etc.
It's not like all parts of the "system" are always up and working, either.
Let's not forget the ever frustrating and funny instances where modern firearms that ought to be institutionally registered to LE agencies (or security companies, etc) haven't actually been entered into the appropriate data bases. Or the "missing" firearms that may, or may not, have been entered into the state systems as lost/stolen. Sometimes inventories done by LE agencies can be ... interesting.
Balls of fun trying to trace guns.