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, from your store settings, check the Access credentials & integration settings panel to make sure your store is configured appropriately:
Next, check the status of the missing listings on your external BigCommerce/3DCart store to make sure they are:
Not Hidden or Not for sale (3dCart).
Also read our guide on troubleshooting missing listings.