I think a lot with winning battles depends on your odds beforehand. If you enter the battle badly outnumbered and win, you're more likely to gain command. If the enemy has decent troops, a high-star commander, and outnumbers you, you're even more likely to gain command.

If your ubergeneral and select army of elite units puts down a lot of small peasant rebellions, you could win battles all day long and not see any change for your general.

A couple more things to keep in mind. First, Depending on battle conditions (who outnumbers who by how much, etc.) you've got a good chance of picking up virtues to enhance your command, such as expert defender, skilled risky attacks, etc. Second, as with agents gaining valor, the number of battles to gain command rises geometrically. Your first star just takes one victory, the next takes two more, the next four more, etc.

Ajax