There are a few factors to consider when the inventory of products within an FBA shipment is not being deducted:

1.  Default Stock Location: Ensure that the default stock location is correctly set for the warehouse where you want the inventory to be deducted by default. You can check this on the settings page under “locations.” 
2.  Ship From Warehouse: If a different “ship from warehouse” was selected for this specific shipment, the inventory will be deducted from that particular location instead. Double-check the selected warehouse for the shipment.
3. Activities and Inventory: Review the activities of the products within the shipment to see if the inventory was correctly added to the desired store or location. This can help identify any discrepancies or issues related to the inventory management process.

By considering these factors and verifying the settings, you can troubleshoot and determine the reason behind the inventory not being deducted properly in the FBA shipment.