I am having the same problem with a reconditioned Pro-Heat machine. The soap is getting as far as the heater in the centre of the base.
There is a lot of movement in the inlet pipe but not much in the outlet pipe to the nozzles. I have removed the part and blown into the outlet pipe. Some foamy liquid did exit along with some coagulated soap. I have still to refit it and try again from a safe distance. I had noticed some coagulated soap floating in the pipe. I'll feed back if I have any luck.
I was advised by a Bissell repair agent not to buy it :(. I have to say that it does a good job when it does work or is assisted by a watering can of hot water!
David (Glasgow)
May 2011