Board Thread:Dark Souls III/@comment-27041871-20160425213110/@comment-25257440-20160426041109

It seems to me that you are trying to summon in areas you have already beaten the boss in. You cannot do White Sign summoning for co-op purposes in these areas anymore, you have to move to the next areas, as the reason for White Sign summoning is to help the host beat the area boss. As such you can only be summoned in these areas, but cannot summon other people yourself. You can, however, do Red Sign and Purple Sign summoning for PvP purposes in already cleared areas.