boonchoo - I also experienced this some time ago, and what it seems to depend on is whether the ticket has been activated or not as an e-ticket. This will happen when the final payment has been made (when SAS actually takes the money from the credit card).
You can see this information on www.checkmytrip.com for example.
It will be found under the heading "Ticket Issued" where it will display the name of the passenger, the fact that it is an electronic ticket, and the actual ticket number. If this information is not contained in the booking, the ticket does not "exist" yet (has not been activated as an e-ticket), and can therefore not be checked in...