Air trapped in hot water pump

Hi I live in a 1st floor flat, the hot water is stored, and the cold water is from the mains. The flat is electricity only. no gas I recently had my electric shower replaced with a non electric one. As the hot water pressure is low, we fitted a pump so there would be good water pressure. The pressure is really good now and the shower is great. However, there is a problem that the bathroom fitters cannot figure out. My hot water heats up every night on economy 7. (Between 12 and 7 am) Every morning air gets trapped and therefore no hot water comes out of the taps and shower. The air has to be released manually for it to work again. The water works for the rest of the day, and every morning after the water has heated I have the same problem We have tried fitting a new more powerfull pump. But the same problem occurs. Anyone have any ideas, We are all stumped? cheers Ravi