Since all of the doors share the same ground and that is before the relays and the front doors work and there are 2 relays (1 unlock, 1 lock). There are only 2 options ; wires/connectors or motors.

 

Though I did not notice it right away, my 2008 LS power locks were feeble from day I bought it. It was a former rental and when I got it there was some evidence that the locks had already been fiddled with. Eventually they worked inconsistently and ultimately little or nothing and blowing the fuse. I just pulled the fuse and lived with it. The lift gate lock continued to work fine with the button and remote; it’s on a different fuse.

My 2010 and 2011 HHR locks operate reliably with noticeably more authority. The 2011 driver door lock switch gave me some trouble but was easily fixed.

In a memory fragment, on prior thread on this subject, a member found a crushed wire under a seat that proved to be the cause of his locks troubles. I seem to recall his was wimpy operation due to high resistance but of course OP could have a broken wire.

As a first step I would pull the connectors and test for power when operating the locks.