Changing Coffee makes me change grinder level I have a Gaggia Classic with Grinder Rancilio Rocky.

Last week, I changed coffee in my grinder, and now, coffee flow is very low.

I had to change 3 level of grinder for my new coffee...

Why Grinder level is coffee bean dependent?
Expanding comment into a (possible) answer...

On the topic of different beans in a burr grinder: I find that my (admittedly cheap) burr grinder "chokes" on darker-roast beans. When I put dark roast beans into the grinder, the chute gets clogged with coffee grounds more quickly (as compared to lighter-roasted beans). I have a timer-based auto-shutoff model, and much less grounds come out when the grinder is "clogged" in this way. I attribute this clogging to the oil on the surface of the beans that is somehow "sticky" and more prevalent on darker roast beans, but I'm not certain. This article from Coffee Geek talks about cleaning more frequently for darker roast beans, as does this review from Seattle Coffee Gear, as does this one from Coffee Review. 

On the topic of espresso pulled from these grounds: This is tricky and I'm really just speculating. Perhaps it's a similar situation: your darker-roast beans respond to tamping differently than your lighter-roast beans. If you're tamping hard and the grounds are clumping together somehow, you might be getting some channeling (resulting in uneven extraction); this could cause a more "watery" shot. You might be able to see evidence of this in the puck after you pull the shot, or you might be able to see it while pulling the shot if you use a naked (bottomless) portafilter. This article from Home-Barista talks about a too-quick extraction being paired with grind flaws; are you seeing that? It also discusses various other flaws and corrections, too various to summarize here.