You might have enabled the Supply Allocation feature, created Allocation Strategies, set Demand Preferences for Supply Allocation, however, the Allocated Supply column in your transaction (e.g. Sales Order) is still 0 even if there are quantities On Order.
One possible reason why the Supply will not be allocated is failing to populate the Location in your Purchase Order. When this is the case, edit the PO and set the appropriate Location. Refresh the page and you should be able to see that the quantity is allocated.
However, if the Location is already populated and Allocated Supply is still at 0, the Expected Receipt Date might not be visible on your Purchase Order form. When this is the case, customize your PO form. Under Sublist Fields, make sure to mark the Show checkbox, and Save.
After showing the Expected Receipt Date field, you must set the date in your PO to set the Allocated Demand.
The Allocated Supply in your Sales Order should now be showing the correct quantity.
Remember that this feature is to efficiently match supply with demand, thus, details for both supply and demand transactions should be checked.