Position extraction not working on 3rd party PDF

Note: I saw this forum post but it wasn't really giving an answer so I am starting a new one to see if we can figure this out.

Background
We have a PDF we are given by a different company/system which we have no control over.  It has fields within the PDF which have specific names. We are trying to use the OneSpan position extraction feature to locate the named fields and apply a signature to them.  Unfortunately with this PDF it is not working, however with other PDFs we are able to make it work.  We are hoping you can help us understand what the requirements are for a PDF in order to facilitate position extraction.

Issue

See attached images for more details!  The PDF we are trying to upload for use with position extraction has a "Text Field" within the PDF with a specific name of "DocuSignSignHere_C1A".  The version information on this PDF is 1.3 (Acrobat 4.x).  It was generated by BFO.com.  When we attempt to use the following code, however, OneSpan seems unable to find the field and apply a signature field to it.  I used a PDF from one of your examples as a secondary test using this same code (albeit it a different signature field name) and it works perfectly. 

So my question to you is what are the requirements of a pdf and/or field in order for position extraction to work?

Code

PackageBuilder
                .NewPackageNamed("Test Package .NET")
                .WithSettings(DocumentPackageSettingsBuilder.NewDocumentPackageSettings())
                .WithSigner(SignerBuilder.NewSignerWithEmail("[email protected]")
                    .WithFirstName("John")
                    .WithLastName("Doe")
                    .WithCustomId("Signer"))
                .WithDocument(DocumentBuilder.NewDocumentNamed("First Document")
                    .FromStream(fs, DocumentType.PDF)
                    .EnableExtraction()
                    .WithSignature(SignatureBuilder.SignatureFor("[email protected]")
                        .WithPositionExtracted()
                        .WithName("DocuSignSignHere_C1A")))
                .Build();