When Julia was trying to cure Barnabas, perhaps the writers hadn't thought of Angelique yet. Or maybe she was just too busy serving time in hell.
We're speaking fictionally, of course. I prefer to suspend disbelief, and operate mentally in DS reality, not "real reality" (which is often more unbelievable to me anyway.) I think it makes sense to see Ang as having had her history changed by Victoria's presence, how we don't know, and in the reality that existed prior to the seance, Ang had not been alerted to Barnabas ever having been let out of the coffin in the future, or treated experimentally by Julia.
I suppose Ang could have picked her own point in time to shoot for, once history
had been changed, even a point before the seance (if there's such a thing as a changed-history seance where even before the seance began, Vicki had been in the past, since she'd created a new timeline in which 1795 included her, from 1795 on, including every year up through 1967 and beyond)... but then it all becomes a mess that I give uip on figuring out. But if Ang had tried to come before the seance, to stop Julia's experiments, well, Julia wasn't exactly getting anywhere with them. She wouldn't be seen as much of a threat to the curse. If Ang can go shopping amongst all the various points in the future timeline (which I doubt, she isn't all-powerful), she'd know Julia doesn't succeed, and that Lang's the real threat to the Curse.
A much simpler way to look at it.... Somehow, Vicki's blatherings about "her" Barnabas mad their way into Ang's ears, not sure how... and when Vicki was catapulted back to 1968, this left a sort of trail in the "ether" perceptible to Angelique. Ang followed Vicki's time-trail to 1968, and only then started to observe everything going on... and she got there before the treatments probably, right behind Vicki in time. She only knew about Lang and the "cure" by seeing it unfold, after Vicki had led her there.
Note, Ang knows nothing about Julia in 1968 until she questions her. Ang wouldn't have known to pop in during 1967 to stop Julia.