Hi Steve first of all, the electric shower must be fed by the water mains at high pressure.
Next, if the upstairs taps are fed by a loft tank, they need to be ideally 1 metre below the bottom of the cold tank to give reasonable flow.
So, either the tank isn't fully filling with water - which is easy enough to check - or you have an air lock in the system. To clear the air lock - which usually only occurs on the hot side - you connect a hose pipe to a mains pressure tap, then connect the other end of the hose to the upstairs tap, and turn that one on. When you then turn on the mains tap, the water rushes through the hose, through the tap and pipework, back to the tank clearing any air lock as it does so. How you prevent water squirting everywhere is another issue! Hope this helps you understand the system.
John
July 2009