A: The basic cause is insufficient flow of water through the shower.
Check the simple things first.
Is hose kinked or otherwise obstructed and is shower head clear.
Check that mains stopcock and any valves in supply to shower are fully open.
Check that inlet filter in shower is clear.
Check that mains water at cold tap in kitchen has adequate flow and pressure and if in doubt ask water supplier to check mains supply.
If all of above are ok then it is likely to be the flow valve at fault and arepair kit can be ordered online by clicking on the link below.

http://www.showerdoc.com/shower-spares/galaxy/GALA-SG06045-Flow-Valve-Assy-
Answered 42 days ago | Alastair