There's a 15 level range you are allowed to plunder so level 59 players and below won't be able to plundered by you while level 60+ will be in your radar. Also you need to be top 20/top 50 (these are based on reports from players) on ranked battle to be allowed to plunder. It could also be that while you were going from konaha to your group to plunder, someone else from a different group waiting to plunder got to it first.