This could be due to a number of things;

• Imbalance of the hot and cold water entering the shower valve
  Thermostatic mixing valves automatically balance the amount of hot and cold water being mixed, which will prevent drastic fluctuations in shower temperature. If the cold water flow is reduced (due to a toilet flush), the valve automatically adjusts the amount of hot water is mixed. If it’s starting to fail which would be evident by having to turn the valve to a hotter setting (say 45 degrees) to get 38 degrees then the valve is starting to fail and would need to be replaced. 

• The water valve is not thermostatic, it's a hot and cold mixer.
  In this case, we recommend that you fit a Pressure Equalising Valve (PEV) to the rear of your shower.
  
  For more information on this, search our knowledge base.

• Gravity feed system doesn’t have enough pressure.
  Homeowners with gravity fed water systems can often experience low water pressure in their properties. If you want to see if you have this type of system and some solutions to the problems of low water pressure then, WaterPressureProblems.com can help.