From the DDI compendium:
Effect: You mark the target. The target remains marked until you use this power against another target, or if you fail to engage the target (see below). A creature can be subject to only one mark at a time. A new mark supersedes a mark that was already in place.
While a target is marked, it takes a –2 penalty to attack rolls for any attack that doesn't include you as a target. Also, it takes radiant damage equal to 3 + your Charisma modifier the first time it makes an attack that doesn’t include you as a target before the start of your next turn. The damage increases to 6 + your Charisma modifier at 11th level, and to 9 + your Charisma modifier at 21st level.
On your turn, you must engage the target you challenged or challenge a different target. To engage the target, you must either attack it or end your turn adjacent to it. If none of these events occur by the end of your turn, the marked condition ends and you can’t use divine challenge on your next turn.
You can use divine challenge once per turn.
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The bold text should solve your problem.
If you use DC against an enemy with your minor action and that is your last action for your turn, you must be adjacent to the target of your DC or " by the end of your turn, the marked condition ends and you can’t use divine challenge on your next turn.".
If you attacked the creature prior to your marking and you are not adjacent at the end of the turn that would void your DC and you couldn't use DC next round.