Thank you for your question as to why the garage door will open all the way but slides back 6 inches.
Garage door opener why the door slide back down.
Check to be sure these two sensors on either side of your garage door.
If you garage door is lowering then reversing back to the open position there could be something obstructing the sensors.
The one that is not is not strong enough to hold door up on its own but strong enough to still open the door.
You will have to take the cover off the motor unit to check the gear.
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.
It will be obvious if it is going bad.
You need to check the door springs and the drive gear in the opener.