The reason Sarah did not appear in 1897 was that Barnabas did not summon her, the way he unconsciously did in 1967. In 1967 he was being Evil Barnabas, and his good self, which had not been totally extinguished, struggled to express itself and finally did so by calling forth Sarah's ghost.
I'm very impressed by this new insight, Lydia! Thanks for sharing!
I'd never thought of it that way before.
While we're on the subject of Sarah, I was disappointed that she never returned to Barnabas, after promising him she would only appear to him once he learned how to be good. Maybe she didn't think he had changed much over the years? :-
It would have been a fitting end to the show if she would have come back, saying how proud of him she was for partially conquering the evil nature brought on by the vampire curse, and that she forgave him.