to make sure whether Is the cold water pressure low high enough. We should ensure enough water pressure more than 0.1Mpa (10m)-, so that get good flows, because hot water can flows only due to inlet pressure.
To make sure whether Is the water inlet valve on the inlet line opened to the maximum. you can adjust the water inlet valve to get what you water of water outlet.
The nozzle may be blocked by impurities when it has been long time used. a cross screwdriver may be used by the user to take off the middle screw of shower head. Then purge the inside with user's tooth brush and detergent.