How to Manage Shipment Allocation
You may try to allocate a product to a shipment but find that the system shows as having zero of this product. This could be genuine and there may indeed be no stock in the warehouse. However, if you know there is stock which you should be able to allocate then you may need to Re-Calculate the inventory.
To do this you need to go to Inventory > Stock by Product, find the product and highlight it.

At the bottom of the page there will be a button titled 'Recalculate.'
Press this and the system will perform a check, calculate the stock for this product and update the figures.

This will then allow you to allocate the product if there is a valid amount in the warehouse.