I booked a flight on the KLM site several months in advance and got the line: "U kunt niet online inchecken voor deze vlucht."
(You can't check in online for this flight.)
The flight is on a national holiday and I was pretty much sure at the time that the reason for not being able to check in online was that holiday.
This is a standard flight, booked on the KLM site directly, by someone who is a Dutch citizen and who has checked in online with KLM several times in the last few years.

Is there an other option and still be able to check in more than three hours before the flight?