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.
It will be obvious if it is going bad.
We recommend unplugging the garage door opener and then replugging it back in after any repairs or adjustments to reset the system.
You need to check the door springs and the drive gear in the opener.
Check to be sure these two sensors on either side of your garage door.
You will have to take the cover off the motor unit to check the gear.
It can also be caused by dirt on the sensor eyes loose or corroded wire connections at the sensors or the motor head or just that the sensors have gone bad and need to be replaced.
If you garage door is lowering then reversing back to the open position there could be something obstructing the sensors.
Garage door opener problem.