If you can, inquire about why they're passing. Sometimes the timing is wrong; sometimes they're interested but have a concern about whether you'll be able to accomplish something or overcome a particular obstacle.
Begin by asking if they know anyone else who might be interested. Then ask if it's OK if you stay in touch or, alternatively, just add them to your e-mail update list (and remove them if they ask). If you are having an open-house, release party, etc. invite them.
It's never a bad move to call or e-mail once in a while with a follow-up question about something that was said during your conversation or something they may know about.
I've been working with young companies for nearly twenty years and learned I never know when something good will come up unexpectedly. This can include an introduction to a customer/vendor/investor; a check in a later round from someone who passed earlier; or even a bit of insightful help or advice.