Check with PowerGear, as Fred suggests.
I see two possibilities: either the valve in the pump isn't opening for that jack OR the controller isn't engaging for that jack. You didn't mention which mdel PowerGear system you have, but in most the rear jacks are separately controllable and I think the console switches is where the two are ganged togther. Might just be a bad switch or loose wire.
There is an outside chance that something has jammed or damaged the jack itself and there isn't enough pressure to force it down. It's a pretty powerful ram, though, so that is unlikely. You would probably notice it if there was that much damage.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC