This happens alot to me in the mid- to late-game stages. I'll have pleanty of weapons and horses, I can make a colonist become a soldier, or a scout, but I cannot make him into a dragoon. At this point, the error occurs to maybe 40 or 50% of the colonist from then to the end of the game. I have learned to play with more soldiers than I prefer. Has anyone else experienced this? Am I missing something?