Garage door won't stay closed

My sister's Garage door won't stay closed. It's an automatic. As it travels to near the bottom, it stops and returns to the upward position.It is not obstructed, the channels are clear and the wheels turn freely. I thought it might be a problem with the balance of the door, but it moves fairly easily from top to bottom, and furthermore, it is a single spring setup where the one big spring is centrally located on a shaft that turns both pulleys at the ends evenly. The cable tension seems right and even both sides. Also, it is only an eight foot wide door. Lastly, even with the door detached from the drive (Manual Mode), the carriage will not stay in the down position. I tried adjusting the down force and down position, but it didn't help. There is no infrared sensor. What could be the cause of this problem?