I just bought my 2011 HHR and it has the same problem - did you find a solution to the problem? I do not have a remote for this car so it is all manual. I've tried using the lock/unlock buttons on the driver and passengers front doors but with all the other doors locked the liftgate still opens.

 

Read all the posts and links in this thread. Pretty much all the possibilities and fixes are covered. You just have to figure out which one is your "fix".

 

Welcome to the site! No FOB, maybe the BCM isn't recognizing the rear hatch lock, does it click when somebody uses the drivers door master lock button?