By default, Emacs considers a period followed by two spaces or by a newline as the end of a sentence; a period followed by just one space indicates an abbreviation, not the end of a sentence. Accordingly, the fill commands will not break a line after a period followed by just one space. If you set the variable sentence-end-double-space to nil, the fill commands will break a line after a period followed by one space, and put just one space after each period. See Sentences, for other effects and possible drawbacks of this.