If you garage door is lowering then reversing back to the open position there could be something obstructing the sensors.
Garage door opener why the door slide back down.
Garage door opener problem.
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 need to check the door springs and the drive gear in the opener.
It will be obvious if it is going bad.
Check to be sure these two sensors on either side of your garage door.
Thank you for your question as to why the garage door will open all the way but slides back 6 inches.
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.