Joan also had time off for a play during the Cassandra story, which led the show to create the story of Cassandra/Angelique sending Liz into a living death.
I think Vestoff went to work on the film version of 1776. It was nice she at least got that brief spot in the 1841PT but it was disappointing she never appeared after that one episode.
There were long periods of Karlen barely appearing - he was only in the 1897 story for a few weeks, then somewhat infrequently appeared as Willie near the end of the Leviathan story and off and on in 1970PT (to give a reason why Barnabas couldn't be there, then he was killed off not too long after the filming for House of Dark Shadows wrapped up). I can't remember if Willie was in the Daphne/Gerard haunting story at all. Roger Davis was originally meant to play the role Karlen played in 1840 - that was his first quasi-leading man part and romantic part on DS, and I guess he did well enough to where they had him do the same in 1841PT. I wish they'd given him that opportunity earlier on.
I can't remember whether it's true or not but I swear I read David Henesy left the show and he and his family moved to California in the middle of the 1840 story. That would make sense, as recreating the Quentin/Beth haunting story with David and Hallie Stokes in pivotal roles quickly fizzled out once they got to 1840. Hallie/Carrie then had some airtime with Tom Happer - he was the son of a Thayer David character (Mordecai Grimes) who was some sort of farmer or something, and the two had a romance. Bless her heart, Kathy Cody could not act, and her airtime was very limited. I think in 1841PT they had her 1840 character's double as having psychic powers, which at least gave her something different to do.