Streetfield BMW

Sorry it's an issue with Rear Air suspension on a 5 series tourer

Hi I have 2014 plate 3.0d 5 series tourer. Over last couple of months the rear air suspension has played up. It has on a couple of occasions not raised the rear end up on starting the ignition. The wheels sit on the bumps and this is obviously a problem. The suspension then works if I stop the car and restart engine. It holds the height when lifted and rear end is even. Compressor doesn't seem to overrun once rear is lifted and I can't here any hissing noises from air bags leaking. Car has high mileage but issue has only started recently. Question - how should the system work? Should it keep the rear end permanently lifted and never drop down or does it do this in certain circumstances such tailgate open, fully loaded to ease access?? Any help suggestions appreciated. Thanks