I've only found this if the tablet has been put in the dispenser and left there for a while before switching on.....try using the new type which dissolves the plastic coating - they are liquid inside? Normally you can hear a clunk after a few minutes of switching on - this is the dispenser door opening. I guess you should have a look at the slots on the spray arms for blockage though - its the water that washes the tablet down and any restriction in flow could cause what you see.
John
December 2007