Did you use proxies with those accounts or did you use all of them on the same ip? How fast did you join groups? Did you do anything else with those accounts to make them look natural, like adding a couple of friends, liking stuff and so on? Facebook has become more and more strict and you need to make your accounts looks as legit as possible, just getting some fresh accounts and starting to blast with them won't work anymore, you need to hide all footprints that you can.