The most common reason for this is if the name booked is slightly different to the name in your passport. (eg Booking in name of Smith John because the traveller calls himself John Smith, but Passport issued as Smith, Ian John because that's the order his mummy and daddy registered him as!)

That will cause APIS to throw a bit of a hiccough, but behind the scenes, a human interface can spot immediately what is causing the wobbler - hence the appearance of the green tick.