The ability to run the ball well is not important when we're talking about play action, only the threat to run. If you're running the ball down the defense's throat then of course that helps some. But you don't even have to be a good running team to run play action well. You only have to be good at selling the defense on the run.
There were a lot of times when Schaub was here that we would have a bad day running the ball, but he would still be able to bring the defense up on the fake. Not because they were scared of giving up rushing yards, but because they believed the play was a run and reacted to that.
I don't know yet if it's our QBs, our OL, or even our scheme, but the defense isn't buying it. No matter how well you run the ball, play action only works if the defense believes that you are running it on that play. Foster could have 200 yards and we still wouldn't be able to run play action, because the fake itself isn't faking anyone.