It could be a number of things; some key questions:
* How far apart in the funnel are your clicks vs your impressions? What’s the CPC? That can help you figure out if your cost cap is reasonable.
* Have you checked that your conversion is firing? (Go to the Pixel option in the ads dropdown; you can check if each of your conversions are firing there.) If you’re targeting conversions, but they don’t fire, FB just won’t run any ads.
* You’re using the same exact creative, I assume?
It may simply be that you need more traffic to optimize for conversions. If that’s the case, optimize for clicks for now; then switch to conversions once you have more traffic. (You need about 50 conversions/week to effectively optimize for them.)