I have had a similar problem for many years. It appears at random and seems to go away if you leave it for a while (anything up to 24 hours). I have taken the machine apart and have failed to find any physical reason (blocked pipe, jammed pump etc.). However, patience prevails. Even if you try to start the programme loads of times it seems to make no difference. I've not yet found a fix and the machine is really good when it is working so I have learned to live with it. If anyone has a fix I'd be interested.