I have a secondhand Gaggia Classic that I have been brewing with for about 2 months without any problem. One day the pressure fell to only 3 bars, something I fixed by dismantling it and cleaning out the dispersion plate. The pressure is back to normal and I can brew a good cup with a bottomless portafilter. However, when I try using the standard portafilter it seems as if there is not enough pressure. I do not pack the coffee very hard (same pressure as before), but it can still take up to two minutes to get enough water through for an espresso.
Any idea what the issue can be?