air suspension works sometimes it does sometimes it does
 

I have a 2004 audi a8 the front air struts are not working. On some days they come up find and work for a few days, then they go down and will not come up for a few days. The struts seem to be find as well as the pump so what can be causing this problem. if anyone can help me with this I would really appreciate it thank you very much CJ.