It’s been a mystery to me for a while why some Shopify purchase events get successfully tracked via FB Pixel/CAPI and some never get matched to FB users. Some days, as little as 1/3 of all purchases would be tracked by FB.
Sure enough, even with the maximum data sharing (email, phone, ZIP, etc.) between Shopify and FB being enabled one should expect a certain % of purchase events won’t get matched to the respective FB users. However, just recently I noticed that the majority of all non-tracked purchases were those made via PayPal.
Are you using the standard FB pixel implementation for Shopify or are you implementing through GTM or hardcoded into the .liquid?
Depending on implementation sometimes the paypal, amazon, etc. third party payment options won’t track because they dump off site to process and then back to the shopify store. So that’s the first place I’d look.
Second would be to make sure you don’t have any extra code or events happening on your cart / checkout / confirmation page that could be impacting tracking or visibility. Sometimes happens in some themes.
If neither of those solve it, go run some dummy orders with paypal and see where the dropoff is happening on the pixel event tracking. That’ll likely point you to the problem area and at least give you somewhere to dig in further.