Line Spacing in Twenty Twelve

https://wp.me/p3LMKS-i

I’m using the Twenty Twelve Theme, visual interface, and can’t get any of the content in my post spaced. It doesn’t matter how many times I hit return, the whole page’s formatting always bounces back to single line spacing.

I’ve tried addling </p> and

in the text but it doesn’t do anything, just disappears.

Please help! this can’t be right, there myst be something I’m missing in the visual, it’s such a fundamental formatting issue.