This sounds to me like pages that have been overridden. Once a page is overridden, Dorico won’t automatically move frames (or flow headings) if the casting off changes; it assumes that if you’ve manually adjusted frames then you know best, and you’ll want to hang onto the adjustments you’ve made.
Remove the overrides (see Can't Remove Flow Titles :-( - #2 by pianoleo - I draw the line at writing it out twice in 24 hours).