There is something to remember about conquest, and it can bite you if you are not careful.
The normal sequence of play to get a minor is thus:
1) Declare war
2) Defeat forces in the field
3) Defeat city garrison in capital city
4) Wait until the land phase in the turn FOLLOWING the turn in which the garrison was defeated
5) Conquest occurs automatically at the end of the land phase
Now, please note that there are two other situations that can happen. The first is between 1 & 2 or 3 & 4: If another nation, who happens to be at war with the controlling MP of the minor, goes into the minor and conquers its capital, then it is a toss-up which power will take the minor. The game seems to roll randomly.
Note that it is possible to move one's corps counter out of the minor during the turn that occurs in steps 4 & 5. Since the LAND phase is the one in which control is determined, this occurs the same turn. "Lapse of war" only occurs in the diplomacy phase, so it cannot happen between steps 4 & 5.
The second way is much more important, and seems to be the one likely to have happened to you. In this one, there is a turn between steps 2 & 3 (i.e you failed to breach the city defenses, or lost the combat). During this turn, if the controlling MP moves before you do in the land phase, then the garrison can die of starvation during foraging. If this happens, it will APPEAR to you exactly the same as if you had conquered it the previous turn. However, you have not had control for the WHOLE turn yet; the city was not denuded of troops until the land phase (of the other major power).
If you move your corps counter out during the same land phase when the garrison died of starvation (or, died to another MP's forces) when an at-war turn came before this one, you will NOT capture the minor (because you didn't have control for a WHOLE turn). Instead, there will be a lapse of war in the next turn's diplomacy phase.
This second one has bitten me I don't know how many times in the computer game. It's because I don't remember accurately whether I owned the capital at the beginning of the turn.
So, I've come up with a way to keep track: I place a factor into the capital city ONLY after I have breached the defenses and won OR if it is the land phase and I now control the capital city. If I see a factor in the capital, then I know that it was placed there on a turn previous to the current one, and I can safely move my corps counter out of the minor's territory.
I think the game designers might consider adding some aspect to the minor nation that would tell us if we owned the nation's capital at the beginning of this turn. Perhaps change the color of the border or of the capital or something, a change only occuring at the beginning of a turn when the conditions apply.
At LAST! The greatest campaign board game of all time is finally available for the PC. Can my old heart stand the strain?