You need to qualify also with some MINIMUM stats too besides XP. This is a feature of the FREELANCER mod bundled within Brytenwalda, and as a rule of thumb try to aim for around 12 on physical stats. This will not be a problem unless you focus on a pure cha or int char.

You ALSO, depending on the type of troop you are being "promoted" to, need a MINIMUM of certain points in particular skils like Power Throw or Ride, around 1 to 3 usually, and as a guideline check the troops skills on the Troop Tree