Only few sales orders backordered

Hi Freinds,

In UAT instance only few sales orders are backordered even the follwoing conditions are satifieng.

1: onhand fine

2:staging sub inv, locators are defined

3item , sub inv, locator relations are defined.

4:periods are opened

5:Shipping parameetres are defined.

5:i tried to relase the orders by using rules and with out rules--in both cases sales orders are back ordered.

6:not able to see any the log file but pick list generation concurrent program completed with waarning BUT for all other sucessful released orders the same program compled normally.

Can you please let me know why only few orders are backordered.