There are usually two culprits here. 1) The piston of the Pavoni screws onto the lever shaft. Over time I have seen these unscrew themselves and the piston will drop down and cover the water inlet hole. To fix you can simply remove the grouphead pop off the dispersion screen and then screw it back on. I think all the steps are in one of my early posts where I show how to replace the gaskets. "
Refeathering the Peacock". 2) If you have taken the arm off, one of the common mistakes is to put it back on upside down. The handle holes are slightly offset...thus if they go on upside down then they will not lift the piston to the point of fully clearing the inlet hole. Thus it is important to watch the orientation of the handle when removing it. I would also take the time to give your dispersion screen a good cleaning and possibly replace the piston gaskets. After 4 years, I am sure they need it.
There are a couple older sites that have good information on the Pavoni, but we have a great group of owners out here who can help you with about anything that comes up. Thanks for joining.