The one that is not is not strong enough to hold door up on its own but strong enough to still open the door.
Garage door opener why the door slide back down.
Thank you for your question as to why the garage door will open all the way but slides back 6 inches.
You need to check the door springs and the drive gear in the opener.
It will be obvious if it is going bad.
Slipping down a few inches after opening posted by nashbamafan on 5 10 16 at 12 35 pm to sicboy probably have 2 springs above door and one of them is broken.
Garage door opener problem.
We recommend unplugging the garage door opener and then replugging it back in after any repairs or adjustments to reset the system.
You will have to take the cover off the motor unit to check the gear.
Door sliding back down once the door is up all the way it s resting on the horizontal parts of the track and the resistance of the motor to turning because it is low geared should prevent it coming back down.