My house has a built-in wardrobe with bi-fold mirror doors. Now one of the doors is not able to close cos something is wrong with its top fixing: it cannot hold its position on the track, when I try to close the door, the fixing will move and the weight of the door makes it tilt. I checked the screw and it seems tight. I am not sure how the door is kept in its position on the track. What is the use of the spring? excuse for my ignorance. Any tip would be much appreciated. Thanks.