Your self priming valve ( spv) is probably at fault, part number EF0099, when cold and un-plugged, remove the lid and you will see the steam valve on top of your boiler, attached to this on the side is the chrome spv, remove the valve then use a 10mm spanner and undo the spv, inside is a small black ball. Remove this also and clean thoroughly. Put back together and you should be running. If not seek out a gaggia manual service engineer