Make sure you have entered the correct selection values, e.g. Purchase Order Number, Material or Customer. When this does not solve the problem, check the ASN History on the Purchase Order, you might have created an ASN with the wrong Quantity in the past (e.g. an ASN fulfilling the full remaining quantity of a blanket order). For instructions on how to create an ASN, watch the Creating ASNs tutorial or read the relevant section in the quick reference guide