Pls make sure you have the following in place
1) Periods are open for Inv and PO
2) Onhand Qty exists for the Ordered Item
3) Shipping Parametres defined for the shipping Organization
4) Pick Slip Report added to the Document Set and this is given in Shipping Parametres/Release Rules.
Try the following..I think for some last options.
1. Subinventory specified as release criteria, and the order lines do not have a subinventory specified.
2. A specific sales order is entered, and the scheduled ship date or request date ranges are outside the dates specified in the order. For example, order 12345 was entered and booked with scheduled ship date of Nov 1, 2009; and when pick releasing this order, a scheduled ship date range of Oct 1 2009 through October 31 2009 was specified.
3. Shipping warehouse specified on the Pick Release form is not the same as the shipping warehouse specified on the order.