WRONG NOZZLE

Soap injectors on the high pressure side of the pump are called low pressure injectors. This is because low pressure injectors inject soap into the water stream when the pressure in the hose is lowered, with the flow of water staying high, thus laying soap on at low pressure. 

To inject soap at low pressure you need to lower the hose pressure while maintaining the water flow.  This is done by increasing the size of the hole the water is being pushed through and maintaining the engine RPM. 

Generally this is achieved by installing a soap nozzle on the end of the wand.  Soap nozzles have a hole large enough to drop the pressure of the water going by the soap injector which creates a venturi action.  This sucks soap into the hose which mixes with the water going by.  

High pressure soap injectors inject soap into the water stream before, the pump.  This soapy water is then pressurized by the pump and sprayed out the nozzle at high pressure. thus laying soap on at high pressure. No soap nozzle is needed, just your regular nozzle.

There are many other reasons for soap injectors not working.  Check the index for more information.