Depending on the Pressure Washer you own, the soap dispenser will be located in the front or back.
If your Pressure Washer has a feed tube, it's important that when you put the cap back on, that the tube is pushed all the way to the bottom of the container - otherwise it won't draw all of the soap up.
Regardless of the model, soap will only dispense in low pressure mode.
Ensure you have the lance set to low pressure, and that you're using the low-pressure tip.
Allow up to 45 seconds for the soap to dispense.
Ensure the soap feed tube is pushed all the way down into the container.