air (or very low pressure) in the water inlet pipe - honestly! We had this problem with ours several times. Once was after one of the inlet hoses had to be replaced. The trick was to ensure the hose is primed and full to the brim with waterbefore screwing it on. The other times were following water main work / pressure failure when air accumulated in the inlet hose. Same treatment as above got it working again.
Tom
December 2008