To start, I'd like to point out that I have played both BOA and ACW, so I'm familiar with the game mechanics. I just have a problem with how they're implemented --- or not.
First, the Jena scenario starts in October, and by mid November, the weather is "Harsh / Frozen / Blizzard" every turn until the last. Campaigning in Winter is suicide, even with decent supply lines --- which, as the attacker, you never have. As the French, I barely managed to take Leipzing and Dresden before the snows hit --- strangely, I could never succeed in breaching the city walls, even with several genie units attached, and multiple corps involved. I say strangely, because the AI managed to get a division-sized unit down to Strasbourg, and had the lvl 1 fortress there breached in 1 turn. I sat outside Leipzig / Dresden for 3, with no effect.
Once I took the afforementioned cities, the scenario was effectively over, as the Grande Armee sat in-situ and slowly starved to death. Even though I built a depot in the Plauen (same as in the Tutorial) province to push supply along, it never seemed to happen. An army with zero food doesn't fight --- especially in the dead of winter. (Moving supply trains from Frankfurt to Dresden is a waste of time, as they use up all their supply before they even reach the destination!)
As for replacements: Does this even function in the Demo? I had units sitting in major cities (Kassel, Frankfurt, etc.), with plenty of recruits in the replacement pool, and nothing ever seemed to happen: Regiments stayed red, cohesion not recovered --- this was with units in Passive posture, over several turns.
If I sound somewhat annoyed, I am: Doing nothing but hit "Next Turn" until the scenario ends, is not my idea of entertainment.
I'm willing to entertain any notions on what might be going wrong here.