If you only enter 1 "ENTER", it normally starts you one a new line, but in wiki code, what is called "white space" is automatically removed. This is why 1 line only shows on the edit page but not on the view page. If you use the code: <br /> you will get a new line on the view page.
When you allow a blank line (by pressing the "ENTER" key twice) it will show on the view page, but this skips a line, which you may not want to do.
If you use a wiki code indent character (putting a colon as the first character on a line) then you will get a new line, but this indents the line, which - again - you may not want to do.