Not sure if it is a bug ore if I made a mistake:

Problem: "Capture"-button dont show up in CtF-Standard-Game.

Here is a screenshot from a (short test) game. Victory "Capture the Princess" is "on". I was looking for the princess unit in editor, it seems okay so far. The horseman (Reiter) also has "capture" flag on. But no capture button is showing up in game. (I destroid the AI capital and get message "princess captured". Now the Princess is mine, but cant be moved to my capital.)

I think it is something similar to capturing a relic - try placing one of your men on the same tile and look for a special button (don't really know what it is called in german though)

I had a situation in the middle ages scenario, when I captured the Holy Grail, but could not move it - I moved a berserk on top of the unit, and got a seize relic button. Hope this helps :)

It may have something to do with the "already attacked in this turn" flag. If the reiter/horseman that you attacked with to capture the city/stack is already sitting in the square but with one out of its two movement points left, then that unit may still activate as being able to move but it will not be able to attack again or complete the capture action. This is just a guess as to why the capture ability is not always visible for units that may have already attacked somewhere during that turn.

My guess without examining the raw code is that the capture action is somehow interrelated with the ability to "attack". But this is just a guess.