Reference
Why a VIN won't decode
A VIN that returns nothing is usually not a broken tool. It is usually a vehicle outside the database, or a character read wrong.
It was built before 1981
The seventeen-character standard was mandated for the 1981 model year. Before that, manufacturers used their own formats of varying length. A 1974 vehicle has a perfectly valid identification number that is simply not a VIN in the modern sense, and no VIN decoder will read it.
It was not built for the US market
NHTSA's database is populated by manufacturer filings required before a vehicle can be sold in the United States. A car built for the European, Japanese or Australian market was never filed, so there is nothing to look up — even though the VIN is real and correctly formed.
This catches most grey imports, including the 25-year-rule JDM vehicles that are increasingly common in the US.
A character was misread
The most common cause by far. The usual culprits are 0 and O, 1 and I, 5 and S, 8 and B, and 2 and Z. Windshield plates behind angled glass in bright sun are genuinely hard to read.
Run it through the check digit validator first. If the check digit fails, you almost certainly have a typo rather than an exotic vehicle.
It is a partial decode, not a failure
Sometimes the vehicle is found but many fields are empty. That is not an error — it means the manufacturer did not file those particular attributes for that VIN. Older vehicles and low-volume models are filed less completely. What is returned is still accurate.
It is not a vehicle VIN at all
Engine numbers, chassis numbers on some imports, and trailer serial numbers are all separate identifiers that look similar and are not interchangeable with a VIN.