What prevents to release PO,while Q-Info is not Maintained for any particular Material
If Quality Info record not present then you can not save PO.Release is next stage.
Now In Quality Info Record if "Release date : is not prior to creation of PO then it can block PO.
Second thing like If someone has deliberately blocked the QINfo for some purpose then also PO can block.
If Quality assurance agreement not present then also PO can block
If Quantity mentioned in the Quality Info record over then for reset of the QTy also PO can not be created.