eBay has some unique policies regarding product groups. Once you have a product group listed, you can't edit their variations, and you can't add a product to the product group.
If you've recently edited a product group, or tried to add a product to a product, and are seeing errors* on eBay, that may be the cause. Here's to correct it:
End the listing on eBay through your eBay partner portal.
Select a SKU from that product group in your Zentail catalog.
Click the Advanced Options on your eBay card on the left hand side.
On the column that slides out, click "Update & Push Entire Group" at the bottom.
*Some errors that might show up would be:
Duplicate custom variation label.
[ERROR] updateSkuForGroupListing (31 minutes ago)
Variation specifics cannot be changed in restricted revise
[ERROR] updateSkuForGroupListing (31 minutes ago)
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