If the height lamps flash normally,but the car wont come up then there is no air to lift it.Worn out compressor,leaking springs/pipes or any thing else that will stop it from building pressure.Does the compressor run,with the engine running and a door open the compressor should be able to fill the tank and switch off in about 8-10 mins.If it just keeps running then its probably a worn seal.When you close the door after running the compressor the car should then lift.Leaky exhausts can melt the airlines to the rear springs,(the rear lifts first)and just dump all the air to atmosphere.All this can be checked without Testbook - the EAS ECU may not even have logged a fault yet if the system is still trying to work,unless it has logged rear valves stuck shut,which means it hasnt seen an expected height change.Worth having good look round first before you hook it up to Testbook.