I had a similar problem with mine. I pulled the lever on the side and underneath the seat but it didn't move. I took it to my dealer who re-oiled the runners, saying it was dry and it has worked... mostly. Still sticks some times.
Have a look at the runners though. You might see a small bracket on it which has been left behind as the chairs gone forwards, preventing the chair from being operated by the handles. If so try sliding the chair back into the bracket that is if you can move it.