AFAIK, the Army of the Potomac didn't really do much. I actually wasn't aware of any post-Antietam battles of that season. I looked it up, and discovered exactly on battle,
Shepherdstown. Basically, he sent a single corp over (the relatively undamaged Vth) to do what damage it could against a moderately well defended Confederate position. What happened was then a tactically small-scale disaster (could've been a lot worse, of course, but the Confederates were too demoralized and damaged at this point to really take advantage of the battle). The loss of the battle is not really McClellan's fault outside of perhaps not reconnoitering and planning a bit better (although I think even this is just a case of hindsight being 20/20), it's the fault of various ranked subordinates.
Wherever the fault of Shepherdstown lay, the resulting clusterfuck is entirely McClellan's. There was a relatively weak and demoralized Confederate force blocking a potential beachhead from Porter. So, what did McClellan do? He sat there. He didn't try finding another spot to make a beach head (probably the smartest move, depending on the geography) so he could engage Hill and Jackson safely. He didn't try sending additional artillery over to help force the Confederates to keep their head down.
He sat there. Absolutely convinced that the ANV was still massive and ready for round two, he sat there, doing nothing, until finally he was relieved.
That's not really the way a good general should fight.