Three tired pumps in a row? There has to be another fault somewhere. That's too much of a coincidence.
^^^ Wot he says ^^^
So where... This pump works and lifts the car, just not to the extreme. But, thinking about it, does the Estate go really high like the saloon?
It should top out, to the extent that the socks will not extend any more. If the pump is still running, the car should still rise to the top out point.
Just to give an idea.... If you lift the car by hand (I know you can't with your back ) there should be no more travel left in the suspension. The pump will not necessarily stop immediately it tops out.
Put another way, if the pump is still running when its stopped rising, and it's not topped out, there's a problem.
Either something is fouling the suspension, which I doubt or you'd feel it when driving, and it topped out ok when fitting the shocks...
...or the bladders are not receiving enough air pressure.
Tired pump, leak, restriction in the pipes.