Talk:Combined Fleet/@comment-210.195.217.246-20150206170313/@comment-5380363-20150212052125

Check your minimum requirements. It's listed on the Winter 2015 Event page.

You might want to check if your fleet slots are full (minimum 5 free ship slots available). If anything, that event lists everything you need to know about the current events. There's also a 75% sortie win rate minimum as well, though I doubt that's an issue for most people.