Which certificates are required for iText.Signatures.OcspClientBouncyCastle GetEncoded - pdf

Question

I want to set the ocsp when signing pdf document using iText
The ocsp is created using the following code:
OCSPVerifier ocspVerifier = new OCSPVerifier(null, null); // can be null based on https://stackoverflow.com/questions/40765907/itextsharp-ocspclientbouncycastle-constructor-is-deprecated-whats-the-replacem
IOcspClient ocspClient = new OcspClientBouncyCastle(ocspVerifier);
var test = ocspClient.GetEncoded(rootCertificate, cert, "http://my-url.com");
The result of this code is null.
I guess it's because i set the wrong certificates.
The first parameter is named checkCert and the second one issuerCert
Note: I'm using iText version 7.1. It seems the names were changed to checkCert/rootCert in version 7.7
What exactly are checkCert and issuerCert?
I tried with the signing root certificate as checkCert and the signature certificate as issuerCert but it doesn't work.
Any idea or explanation which certificates are required here?
Edit:
The following is the code which i use to create the pkcs7 container which should include the ocsp:
OCSPVerifier ocspVerifier = new OCSPVerifier(null, null); // null,null >https://stackoverflow.com/questions/40765907/itextsharp-ocspclientbouncycastle-constructor-is-deprecated-whats-the-replacem
IOcspClient ocspClient = new OcspClientBouncyCastle(ocspVerifier);
var ocsp = ocspClient.GetEncoded(cert, rootCertificate, "http://www.my.com/aia/ocsp");
if (ocsp == null)
Console.WriteLine("oscp is null");
else
Console.WriteLine("ocsp is not null");
//Create the pkcs7 container
PdfPKCS7 sgn = new PdfPKCS7(null, c.ToArray(), HashAlgorithm, false);
byte[] sh = sgn.GetAuthenticatedAttributeBytes(hash, ocsp, null, PdfSigner.CryptoStandard.CMS);
//Load the signature via pkcs11 hardware
byte[] extSignature = GetSignatureFromHashViaPkcs11(sh, pin);
sgn.SetExternalDigest(extSignature, null, DigestEncryptionAlgorithm);
var ret = sgn.GetEncodedPKCS7(hash, tsaClient, ocsp, null, PdfSigner.CryptoStandard.CMS);
No matter how i set the certificates in ocspClient.GetEncoded, the resulting ocsp is always null
Even a more fundamental question: Are the issuer and root certificate the certificates i use for creating the signature or are those the certificates from the ocsp?