I'm at the part where Lee and Mark have to lift up the section of downed fence.
Mark steps over the fence and starts lifting his post. But when I tap on the target on the other post, Lee just stands and watches. The text "Push Fencepost" appears ("Push" in grey text and "Fencepost" in white) but that's it.
I've checked a this section on a couple of Youtube videos and I don't appear to have missed anything. A click on the other fence post's target is all it takes on the PC version, so I suspect an iOS bug.
I'm playing on iPad 3 running iOS 5.1.1.
Okay, at about my fifth attempt at this (I had to rewind that whole section each time) it worked!
The only difference this time is that I tapped on the fence post target immediately. Previously, I had tapped on the zombie's target to get at a look at him first.