Whenever I come across this the reason is as m.a.riosv suggests
If in doubt, check by displaying the formatting marks Writer by:--
Select Tools>Options>LibreOffice Writer>Formatting Aids to get a dialogue where you can choose which marks are to be shown. Check at least "Paragraph end", "Non breaking spaces", "Tabs", and "Breaks".
If necessary, turn on their display by selecting View>Formatting Marks.
You should see lots of paragraph end marks, few or no break marks.
This might seem elementary, but I recommend keeping the marks displayed all the time you are working. They show up some very common slips as they happen.