Check to ensure you are supplying enough water pressure to the shower 91-3) bar.If you have a Combi Boilder, get your plumber to test the water pressures. You need to have between 1-3 Bar at the shower. (ensure also, that you also have a PEV fitted, if required with your shower model)
If you have a Pump, ensure it is rated between 1-3 bar (ideally 2-2.5 bar is recommended). Check that the pump does not contain any air bubbles and is receiving a good flow of water from your tank.
The Pump should be located close to the water cylinder and ideally within a few meters of the shower. Check pipework and hoses are not kinked and restricting the flow. Check the braided hoses: Remove and look inside at the gauze and washer, ensure they are seated correctly and not inhibiting flow.
Ensure there is no debris built up from any new pipework you have installed or that may have flowed to the shower and creating a restriction.Check that no other water using item is in operation in the house, such as a tap, washing machine or dishwasher etc.
NOTE: Exceeding the 3 bar maximum rating of the shower may lead to pressure damaging the shower and breaking water hose connections.