🌟 The first step is always to check your activity log which can be found in the top right hand corner of your dashboard.
The most common causes for a product not syncing are
Product or variant SKU has been manually deleted or updated on either the source or destination store or
Products and variants are using duplicate SKUs
Solution: Manually updating the SKUs in the Shopify admin and then in Syncio use either the Individual or Bulk Mapping Function to connect the problem products.
Alternatively, If you don't need to preserve unique information, use the Unsync and Delete to remove the product and then use Sync Products to reimport once the SKUs have be repaired on the source store.