I just bought a 2005 outback xt wagon a few hours ago, and I am having this issue. I can hear and feel mechanical action trying to lock when I have the latch of the hatch in front of my face when I use the key fob, but no matter if I use the front door trigger or the key fob, the latch will not lock. I also can have all 5 doors shut, arm the car, have the 4 doors lock, and be able to open the hatch with no change to the alarm. Perfect for anyone to just walk up and enter through the hatch.
I did just charge my fancy expensive dry cell battery and put that in, but I think it's old and sulfur crystalized and no where near decent, so that could be my problem. I doubt it though. It has enough juice to decently roll the windows or move the seats at about half speed. The other doors are managing just fine.
Today is day 1 for me for owning a Subaru. I do not know yet the details of the inner workings of this specific car. I would however like to know exactly without any guessing, what the issue is. It looks like other people are having the same issue and no solutions have been posted yet. So one more call for help here. Thanks!