My Power Shower runs out of water

[Fetchez la vache]

No, it's not a euphamism, but I wondered if any plumbers had an idea..

Power shower runs out of water while there is plenty in the tank - currently lasting only enough for 1 1/2 showers (assuming one at a time ), making the poor shower make one hell of a racket (as the pump presumably tries to push air about)

• Hot and cold water is still available from the taps, just not the shower
• No water comes out at all - not just lacking cold, or hot
• We recently had the tank in the airing cupboard replaced with a bigger one, and the system was drained and refilled during that. However this problem was occurring before that as well (if that helps at all - originally thought it might have been an airlock somewhere)

Hopefully that may ring a bell with someone..

cheers