Are you playing stock SH3 or you're using any of the mods. If you're playing SH3/GW1.1a or GWX then promotions are linked with renown and not tonnage. You have to reach a particular renown (current not total) value before get promoted. Have a look at these links they explain everything.
http://www.communitymanuals.com/shii...p?title=Renown
http://www.communitymanuals.com/shii...tle=Promotions