I have the same problem with my Karcher 411A. I am told that inside the m/c there is some type of ball valve which has to be released. To do this,you have to get inside.The catch is that there are special screws holding the casing on; I was told that a screwdriver head 'T15' would do the job but it did'nt.
John L
May 2011
Don't know what make model etc,make sure you have the jet on the end of the lance turned down to low as most wont work with the pressure on high setting.