Missing listings from these stores could simply be the result of incorrectly configured listings on your channel, or a store setting preventing certain types of listings from being imported.
First, go to Settings > Stores, open the store, and check the Access credentials & integration settings panel:
BigCommerce
Import visible products only (off by default): when checked, products that are not visible on your storefront are skipped during listings sync.
Import physical products only (on by default): digital products are skipped during listings sync.
Shift4Shop (formerly 3dcart)
Skip Importing Hidden Listings (on by default): hidden products are skipped during listings sync.
Skip "Not for Sale" Listings (on by default): products marked Not for sale are skipped during listings sync.
These two settings are available on Shift4Shop stores connected through the REST API.
Next, check the missing products on your BigCommerce or Shift4Shop store. If one of the settings above is on, make sure the product is:
Visible on your storefront (BigCommerce).
A physical product, not a digital one (BigCommerce).
Not Hidden and not Not for sale (Shift4Shop).
Or turn off the setting that skips them, then run listings sync again.
Also read our guide on troubleshooting missing listings.