Ok when you get a freezer which hangs around the -12 mark its due to either ice build up on the evaporator or the fan motor inside the freezer not running correctly.
What you need to do is empty the freezer and remove the rear panel in there to access the evaporator. From there use a hairdryer and melt the ice. Then retry it and see how it goes from there. As that should get it back down to -18 again.
But if it happens again then theres a issue on the automatic defrost side itself which could be either the heater or thermal cut out.
But if you don't want to go removing panels then you can do a defrost on it. Turn it of and leave the freezer door open for 24 hours and this will do the same job.
Then see how it goes.
But normal if its only going to -12 or around that temperature its down to too much ice on the evaporator of the fan itself isn't running.
Check them out and see what you find and let me know.