Make sure you have a hard return at the end of the paragraphs and not a
manual line break. Make sure you have your show/hide button on where you can
see all the non printing characters like returns, tabs, spaces. If you see
all that you will notice at the end of a paragraph if there is something that
looks like a paragraph symbol (this represents a hard return) or a crooked
arrow (this represents a manual line break). If there is a manual line break
that means you are breaking the paragraph here but want to continue all
formatting. So if you have 2 paragraphs with a manual line break between
them you can justify one and it will justify the other. But if you have a
hard return (meaning physically hitting the return/enter key on your
keyboard) between the paragraphs then it will let you do different formatting
for each paragraph.
Hope this helps.
If you have a very long document that has a manual line break at the end of
every single paragraph, you can do a global replace of all manual line breaks
for a return and get rid of them all at once.