You open up your rotation too early.
I don`t know how to perform 360's (yet
) but I found this on the web in the past:
snowboard methodology. If you look in this link you find a pro tip that when doing a 360 (FS in this case) it happens that you`ll find yourself always lading a 270 instead off a 360, just as you say. Hi's tip is to stay tucked longer!
Hopefully this tip helped, good luck putting a BS360 down!