I've seen this problem mentioned on the Steam forums too, but I didn't see anyone mention it here, yet. Anyway, the situation is that upon completing the Kaldwins Bridge mission (.ie. The Royal Physician), trying to leave for the Ball (i.e. Lady Boyle's Last Party) from the Hound Pits pub hub (lol), Samuel has no prompt that will allow me to begin the mission.
The other folks who encountered this problem suggested restarting 'The Royal Physician' mission from the main menu and that would remedy the problem, but it hasn't worked for everyone, including myself. I've enjoyed the game thus far, but this has killed it stone dead for me. Is there any way to get around this without starting the entire game again? If not, are the dev team aware of it, & hopefully working on a patch?

Regards,

A Concerned and Despondent Fan.

Ok, folks. I think I have a fix. If this happens to you, simply step off Samuel's boat into the water and let the hag-fish chew on you a bit. Exit the water and now Samuel should respond, to take you to the party. See Bethesda/ Arkane? That's customer support!

Glad that worked for you.