When I export my InD file to an epub, all the paragraphs are squished together, without spaces between. I've tried adding paragraph styles with "space before" and "space after" but nothing works. Adding extra line breaks or hard returns doesn't help, either. There is no paragraph spacing between paragraphs in the epub file.
Space before/after should work. Can you tell us what Indesign version are you using as well as your operating system. If you can share a sample showing the problem we can take a loot at it. You can use Dropbox or similar and post the link here.
It appears the problem was my epub reader, not InD. Calibre seems to render it well. My apologies!