Hi,
I grind my beans on #4 on the Rancilio. Lately I got new beans, but the machine could not handle this grind...I had to grind on #6 and press very light...? What's the reason for that....
Thanks for your help explaining that....
V.

Different beans (blend or crop), different roast, different period of time since the beans were roasted, different storage of the beans, different humidity in the room.
Pick one... or two, or all the above.

And the numbers on Rocky mean nothing to anyone but you. They are not actually a calibration- just a reference to the user... Two "clicks" on Rocky is about 6 to 8 second difference in the extraction time so it is not all that drastic of a change considering all the factors that can cause one to change the grind setting.

ventura wrote:Hi,
I grind my beans on #4 on the Rancilio. Lately I got new beans, but the machine could not handle this grind...I had to grind on #6 and press very light...? What's the reason for that....
Thanks for your help explaining that....
V.

What Randy said PLUS :
Over the past week, the temperature and relative humidity have dropped drastically. Inside, the humidity is even lower due to central heating.

You may want to review Espresso 101: How to Adjust Dose and Grind Setting by Taste and try a wider range on the grinder.

back in '55 we couldn't Trick or Treat Winterpeg due to several feet of snow

ventura wrote:Hi,
I grind my beans on #4 on the Rancilio. Lately I got new beans, but the machine could not handle this grind...I had to grind on #6 and press very light...? What's the reason for that....
Thanks for your help explaining that....
V.

I'm slightly confused, are you getting new beans of the same type (ie, Black Cat to Black Cat) or did you go to a new blend/roaster (ie, Black Cat to Red Bird)?

If it's the former, are you measuring your bean weight before and after grind? I found that if I measure out my beans by weight, I hardly have to switch my grind as the coffee ages once it's dialed in. Otherwise, I agree with Ian that the temperature and humidity probably affected your bean volume enough to make a difference.

If it's the latter, then Randy nailed it.