With the machine cold: The other thing you can do is try undoing the tube pointed by the green arrow and blowing onto it. If you have the tap open and nothing comes out, then you could assume there’s a blockage around the tap area. If you then remove the tap (red arrow) and air comes to the other end, then you know there’s a problem around the tap area.
If all is good, then you have a blockage on the boiler fitting which leads to the pipe pointed by the green arrow.