How can it be that one day I pull back-to-back perfect shots, 14.5g at 25 seconds using a Monolith Flat, then the very next day, with the grind setting unchanged and using the exact same bag of coffee with the same 14.5g, the Olympia jams up and I can't pull the lever hard enough to get more than a few drops before I finally give up? The grind is the exact same, as is my temperature management, puck preparation, and tamping pressure. One day, perfect shots; the next day, complete jam-ups. Frustrated.

Are you sure it's the Cremina and not the Grinder? I'm sure you know this but changes in room temp and humidity has an effect on grind setting. My kitchen tends to get blasted at certain times of the day, month, and year with sunlight. I've noticed that this does impact my grind setting - even during the day. Something else I've noticed is that keeping the lip/edge of your portafilter basket and group head gasket clean is essential for shot consistency. I was getting frustrated with my grinder not pulling consistent shots. Then I cocked an ear towards my portafilter during a particularly bad extraction. Sure enough I was hearing a quiet hiss - the portafilter was not properly sealing. Now that I'm more diligent about this my shots are far more consistent and my grinder doesn't get yelled at.

Could also be the beans. Every day (or throughout the day), you have to adjust your grind as coffee beans are hygroscopic and thus heavily influenced by amount of moisture in the air

Are you ramping pressure at the same rate for every shot? A gentle ramp possibly with a long pause ~2-4 bar (preinfusion) will help with the flow rate later in the shot.

Might it be possible that you get more then 10 bar with the bad shot? Potentially that could lead to the grounds being so compacted that they are clogging the basket.

I don't think it's the beans and I don't think it's my technique....nothing is different. Sure, it's a different day, but only 12 hours difference (more or less) from my last usage. Maybe it's the grinder, but how? Doesn't seem possible on something that grinds perfectly one day, then all of a sudden much finer the next day on same coffee, same amount. If it is, then I'm not sure what the point is spending $3k+ on a grinder. Seems to me it would be more logical to look at where there is more complexity and variability in the process. To me, that seems to be with the operator or the lever machine or both. But I can't figure out what's different about my technique. As far as I can tell, nothing. So that leaves the Cremina or heat management.

All the Cremina does is a) heat water and b) push that water through compacted ground coffee at a set pressure determined by the force you are applying. Heat shouldn't affect consistency and if you are applying consistent force then it has to be something to do with grind size, grind distribution, or the coffee itself.

Are the seals on your Cremina in good shape? If they are not then the pressure may vary which would affect shot consistency.

As a Cremina owner, I've found the machine allows for a very wide range of variability. The cause of variability in my shots, however, is all due to what I do. Preinfusion time, air in the chamber, the rate I ramp up pressure relative coupled with the amount of air in the chamber, all influence how the shot flows. Sometimes I end up with less air (partly due to preinfusion time), there's more immediate resistance in the pull. If I then ramp up quickly the fines jam things up more than other times. Sometimes if there's more air, the immediate pull is more spongy, and my guess is this buffers the ramp up some and the fines jam things up less. If I adjust my pull to things like that I can still get good shots. That's been my experience.

I've experienced something similar, although perhaps to a lesser degree, and only on consecutive shots. For whatever reason (I'm guessing it is the temperature as more water passes through the group, but that's just a wild guess) resistance on the puck can be greater, requiring a longer preinfusion. It's not a big deal for me personally; in fact, the consecutive shots can have more flavor and crema... where's the 'shrug' emoji?

I find some beans need to be dialed in more than others. I can pull an espresso blends day after day without touching the grind setting. Some light roast, single origin beans can be a constant moving target when it come to grind settings, making fine adjustments even after one "perfect" shot. Could it be the beans?