When adding products to the M2E Pro Listing you may face a situation when certain items are not displayed in the grid as available for adding.
This happens because M2E Pro hides products that are already added to another Listing.
To make these products visible in the grid, use the Advanced Filter under eBay/Amazon/Walmart > Listings > Add Products.
By default, the Hide Products presented in the other M2E Pro Listings option in the Advanced Filter is set to Yes. Make sure to set it to No and run the Search.
Once done, the hidden products will be displayed in the search results so you can add them to the Listing.