Markdown treats a single newline as a space, so that already wrapped text doesn't need to be rewrapped. If you want to have each item on one line, some options:

Two spaces before newline

Foo  << two spaces here
Bar

Yields

Foo
Bar

Backslash before newline

Foo\
Bar

Yields

Foo
Bar

Paragraph Break

Most clients will have a "larger" vertical space if you do this. Use a double newline:

Foo

Bar

Yields

Foo

Bar

Bulleted List

* Foo
* Bar

Yields

• Foo
• Bar