Hey ya, welcome to the journey of coffee, talk about unpredictable!
First, your questions:
1. you don't have to have coffee in the hopper, ever. I dose mine PER SHOT with a scoop. In fact, if you wanted, you could remove the hopper and use a cheap tamper or a blind basket to cover the throat
2. just don't keep coffee in the hopper. If entertaining, you could fill it with however much you like and then remove it later that night. Or maybe leave it for the weekend. I don't recommend keeping coffee sitting in the hopper
People will argue that it defeats the purpose of having a manual doser mazzer, which it does, but hey, we want fresh and consistent right?
My questions for you:
How old is the coffee, is it stale? This can make for inconsistent brewing
At what time of the day did you brew yesterday? And what time of the day today? I ask because a brew in the arvo might require a grind adjustment compared to a brew in the morning. especially if your beans have been sitting in the hopper.
Are the beans (in the hopper) exposed to sun?
Where are you located? what is the weather like at the moment?
Which machine are you using? Are you repeating the steps (including warm up time) for each brew?
so many variables! So much great help here on CS to get you dialed in too