water not getting to jets?

I have an Hotpoint slimline 7800 dishwasher which appears to work?
If I press the start button on program 1 the machine operates but the waterjets do not operate unless I fill up the bottom of the machine with water the machine then runs ok going thru until the end of cycle. but when I try to start it again the jets do not operate! either on program 1,2 or 3? If I fill up the bottom of the machine again it works but only the once!!!