Do you put the weapons needed in your Baggage inventory? (which is not your personnal inventory)
all stuff needed for the arming screens need to be put there (horses, swords for non-rifle infantry, smg, rifles, nades, shovels and so on and also AMMO of course
)
example : you put a rifle (any kind, can just be a 'musket') and ammo (put the 10 pack rifle ammo if you want to save better ammo pack for yourself) in that baggage inventory, go to the arming soldier menu, and you can make a rifleman if you have an untrained soldier! et voilà c'est magique
you can access the baggage inventory in the Camp Menu, jut put your weapons and items there