It would appear an adjustment has been made to the VS PVP settings to only award diamonds to the first victor of a VS PVP match every 24 hours. So if player A fights Player B and Player A wins, Player A will receive Diamonds. But if they fight again and Player B wins, he gets a victory, but does not receive diamonds.
Since i had not seen this change announced anywhere i thought a forum post would be in order. I have tested this sufficiently to believe this is not a bug, though it is possible it is if the devs indicate this change was not intentional.
Consider this a PSA