Ok 2 things can cause this. Either the door lock and the wiring has burnt, Or the carbon brushes have worn down.
The first thing to check is the door lock.
Open the door and remove the metal clamp around the seal and unlip the door seal.
Then you can remove the door lock from there and pull it through the gap.
Then from there look at the wiring block on the door lock and see if its burnt, If it has then the door lock will need to be replaced and the wiring block will need to be retagged.
But if the door lock looks ok then you need to remove the motor and at the back of the motor are the carbon brushes.
Take them out and see if they have worn down.
If so then the carbon brushes will need to be replaced and that will cure it.
Its a strange fault as you wont think worn carbon brushes will cause this fault but it does.
So check them 2 out and see what you find as its nealy always one of them that causes this fault.
Let me know what you find.