If your catalogue audience is too small, your ads may not deliver. If this is the case:

• Make sure that you have a reasonable budget and bid when you create your ad.
• Check that any additional targeting isn't too restrictive (for example, you're using a large Custom Audience as an exclusion).
• Check that the inclusion and exclusion rules you've set aren't conflicting.
• Check that the product set ID belongs to the catalogue which your Facebook pixel is assigned to.
• Try to increase retention time to increase your audience size. This is especially important for websites that don't get much traffic. Bear in mind that ads won't be delivered if the audience size is less than 20.
• Make sure the product set has a good number of items as a percentage of the total catalogue. For example, if there are 1000 items in your catalogue, but only 10 in the set, this may affect delivery.
• Check that your pixel is correctly firing on "ViewContent", "AddToCart" and "Purchase" events by visiting the corresponding product page with the "link" value in product feed, checking that the pixels fired with Pixel Helper and comparing the "content_type" and "content_ids" against the data feed used.

* Nguồn: Facebook