I have the same problem and talking to Dagna doesn't help. I've already exhausted all her conversation options. ☹️
And you did her War Table quest?
I looked it up and it is called "The Arcanist and the Fade". It is middle to east in south Ferelden, the picture has a blue background and shows a dwarf in side profile scratching her head.
Perhaps you unlocked the quest but just haven't done it yet?
I am having the exact same problem! I have done Dagnas quest and i have talked to Cullen and still i cant get to Tracking Down Samson i have like 98 points available and people but it still says its busy and it wont let me do anything i have completed into the abyss as well and i have looked online and still no answers by the way i am pkaying on xbox 360. So please help i am goinf inaane!
