I know this is an old post, but hopefully this will help others who were searching for an answer to this problem.
Dragging breaks in "page break preview" does nothing unless you set page scaling (Format/Page/Sheet) to "reduce / enlarge printout". Once in this scaling mode, manually manipulating breaks works as expected.
From : https://ask.libreoffice.org/en/question/45740/i-cant-move-page-breaks/
