Note that for single line spacing, the actual distance between
baselines depends on the font and size of text. Is the problem text
bigger and/or using a different font than the rest of the text in your
document? Does it contain inline objects, such as Microsoft Equations?

Another thing to do is to look for blank paragraphs -- or possibly
other nonprinting characters. Click the ¶ button on the Standard
toolbar to display these. Does that show anything? For more, see
http://word.mvps.org/faqs/formatting/NonPrintChars.htm.