Aqualisa pump runs but wont pump water until head lowered?

Have got the answer from Aqualisa:
This is due to blocked air.
1) with the pump off, unscrew the shower head and place hose in bath.
2) position temp control to cold.
3) turn on pump and wait for any burbling and blowing of air to be flused through.
4) Slowly increase the temperature at each small step wait for burbling air to disipate.
5) finally turn temp to max and allow water to flow.
6) reduce temp to normal and switch pump off.
7) replace shower head.
8) test by switching pump on , water should pump , if it doesnt.
9) reduce the temperature and increase the flow.
10) do this until a steady flow results at all temperatures

Philip Hird
December 2012