Have you checked the flow comming from an upholstery tool or open pipe into the wast tank?
If the machine has little use it won't be scale build up.
Also when the flow control is in the open position, check the flow giong into the tank....put your finger over the elbow where it dumps water into the fill tank. Also the tank filter for blockage.
If you mean flow at the wand, have you checked the (obvious) filters for being made up with carp?
Probably something or nothing if you have changed the pump.