The things to check first are.1 That the main valve coming into your house is fully open. 2 Then check that the valve on the line to the shower is fully open. If these valves are not fully open then there may not be enough water getting to your shower to open / close the safety device that allowspower through to the elements
If these are OK then youprobably need to replace your flow valve http://www.showerdoc.com/shower-spares/mira/MIRA-1563-507-Mira-Flow-Valve-Assembly-1563-507