Verify that the vendor order number is not being sent as duplicated data, meaning that the MARC coded field/subfield pair should be unique to that number. If there are multiple sets of data in the same field/subfiled, the OPAC may not know which data to load. This is especially important to watch for with the vendor order number for overlay, if there are duplicated fields.
For example, the library has the vendor order number mapped to the 024$a field in their Institution Settings. The vendor order number is the field used for matching in overlay. The 024$a field is used for other information as well such as UPC barcode. There is already data present in the 024$a when the record arrives in WorldShare. WorldShare processes the record and creates a duplicate 024$a field for the vendor order number. Then when the record gets loaded into the OPAC, there are multiple 024$a fields that could confuse the overlay. It may be that the UPC barcode value already present in the record from the provider is being used incorrectly by the OPAC as the vendor number.