Cometly does not track anyone who clicked on your ads before Cometly was installed, so if they come back to your store directly (i.e. they did not click on another FB ad to come back to your store) then Cometly would not be able to track any purchases they make on your store.
Keep in mind Cometly begins tracking pixel traffic the day you place the Comet Pixel on your website or funnel. If someone visited your website prior to the pixel being installed, the sale may not track depending on the URL parameters for that specific sale. This is why it can take 4-10 days for the Comet Pixel to give you a fully accurate picture of how your ads are performing.
Also, although Cometly can see every sale that happens on your store, Cometly will only display the sales that come from ads that we are tracking. Since Cometly’s main goal is to help you get better results from your paid ads campaigns, we do not currently track sales or events that did not come from your paid ads.