My Truckforce portable used to do the same thing, so maybe this will help.

Without it running, take a flat-blade screwdriver, and push in the tip on your quick connector that goes to your solution hose. Sometimes, these freeze up, and will not allow any flow, so the machine cannot build up pressure. It might be a safety feature to keep the pump from burning up, I don't know.

I used to see some water come out, or hear the pressure release. Connect your hose, and then turn on your machine and see if you have pressure and flow. To keep this from happening, when you shut the machine off and prior to disconnecting the solution hose, trigger the wand or whatever tool you are using, to release the pressure.

Also make sure the intake side of the pump is clear (hose and all). Some machines have filters somewhere that can clog.

Another thing you can try.........Connect a section of hose to the machine. Plug a disconnect into the other end so solution can flow. Turn the vac motor on and put the hose end into the vac connector of the machine. Cover the vac connector with your hand so you can get suction on the hose. Turn the pump on. If its going to prime it will. If that doesn't work then go back and look at the pump head.

Some of the new machines on the market have a prime valve that does what I described above without having to use a hose. Its all plumbed internally.