Ask HN: Why can't we acquire repeat users?

I launched an MVP. Targeted my target market through Facebook groups and published several blog posts. We drove 2k+ users to the site, several hundred completed the intended behavior.

We haven't focused on re-engaging, because we figured some percentage would do that on their own. Was that a mistake? Does our UI suck? What other mistakes might we be making? Any other advice to help grind our way to product / market fit?

You can view our site at iPrompted.com