We have that on one of our older cars. The seatbelt got caught between door and frame, causing the metal to bend.
The light is triggered by a microswitch on the door (usually in a little black rubbery concertina thing. If that fails to make contact with the door frame, then it won't flip the switch.
We solved it by putting electrical insulation tape over a small coin or other piece of metal, and sticking it to the door, where the switch makes contact.
This makes sure the switch does connect when the door is closed.
You can check if that's the problem by keeping all doors shut, except one, and pressing the little switch thing. You may need to go round all five doors (including the hatchback), before you identify which door is affected.
If more than one door is affected, you may need someone to help you pressing two or three in different combinations until you find the affected doors.
Good luck :)