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This observation has been made before and it generally has been decided that it's because of not saving the changes after each addition.
What you can do as a check is to move your mouse to the corner of the Store widget while in the sitebuilder. At the top left hand corner of the widget as your mouse over it an Edit facility will appear. Click on this and you will be taken to the Online Store dialogue page, then click on Manage Store, centre left. Here you will see any products that have been added to your inventory and they are displayed as a list. If nothing is there then that means they haven't been successfully added.
Go back to adding products by selecting Advanced Products, bottom Right on that page, and enter one. make sure you click on the Save button at the upper right of the Products addition page and then you can go and look at your inventory again. You should now see the added product.
If you need further support with this, please let us know here.