Entering PO Receipts

If there is a PO that is in Approved status, when it is first entered, it is not available when trying to enter a PO Receipt.  Once the PO is dispatched it is then available.  If you do a change order to the PO and approve the changed PO then the PO is available to enter a PO Receipt.  My understanding was that a PO had to be in Dispatched status in order to be able to enter a PO Receipt.  Any clarification would be appreciated.