Just a quick thought...
It wouldn't be the S/L control box thats at fault would it? The one thats mounted on the rear arm?
The sensor detects the hight, and is asking the pump to run, so there is something right at this point.
What needs to happen next is for the car to rise with the pumped air(it's not hence your current problem), THEN as it rises to a point that the sensor says is correct then it stops pumping or releases air. We can't tell if sensor is fully working as the car is not rising.
Your current task is to find where the pumped air, if any, is going to. As Amba said earlier, pull the air connector off the shock body, if you have air pressure there, then it sounds like the shocks are not holding pressure. Replace.
At that point a good test would be to swap your shocks for known good ones and see how the system behaves without spending any money.

If it then transpires the car continues to rise unchecked, as sometimes happens, you may feel it's not worth the cash to pursue. If all is well bung Amba a few quid and your away.Up to you.
