In order to update eBay variation qty from Selro, there should be a SKU/custom label for each variation in eBay. It is very common to have no variation SKUs in your eBay variation listings as it is not mandatory and you have manually created these in eBay. In order for the stok update to work from Selro, please enable SKU for variation listing and enter the variation SKUs. Once this is completed, please remove the ebay listing from Selro and re-import the eBay listing into Selro.