Dear,
Purchase Order-->Goods Receipt/Service Confirmation-->Supplier Invoice entry-->Payment to Supplier.
If PO line item got locked means, buyer don't want to subsequent steps for the same, till you unblock the same system will not allow to do goods receipt or Invoice receipt.
Can check the same by selecting line item and from Environment-->Item changes, will be known who has locked this line, consult them for further details.
R,
Amala