Would probably need to see the document. You would need to check the paragraph settings on each of your Styles (Heading 1, 2, 3, Normal, etc.).

Hope you are looking at your document with the formatting symbols displayed so you can see if you have any stray line returns, paragraph returns, etc. in there. You can also hit SHIFT + F1 to get the Reveal Formatting pane up, and poke around in your doc to see what is going on as you arrow around in the document.