as suggested below, check when the checkout event fires. Plus, here is some more information about the metrics on Facebook.
Facebook's default attribution settings is 1-day view and 28-days after click. This basically means that if a Facebook user clicks on an ad today, go to your website, see a product and then leave AND let's say, after 10 days this user comes back directly to your website and makes a purchase on your website, Facebook will attributes this purchase to the ad, which the user clicked on 10 days ago. So, in the end, a purchase will show up in you Ads Manager reports. However, if you use another analytics (e.g. Google), you will see that the purchase has been attributed to direct source.
It might be also the reason why you don't see the purchase for the specific day via Facebook channel.