OpenSSL Decryption - EVP_DecryptFinal_ex fails

I'm using this decryption function to get the plain text value of a cipher which was encrypted using EVP AES 265 GCM; I can see data in rawOut but ret = EVP_DecryptFinal_ex(ctx, rawOut, &len); returns 0; can you provide any insight as to why? I've also seen sources which do rawOut + len in the EVP_DecryptFinal_ex code, I'm not sure why this would be needed as it would move the pointer to the end of the buffer.

unsigned char* keyDecrypter(unsigned char* pszMasterKey)
{
    ERR_load_crypto_strings();

    int ret, len;
    EVP_CIPHER_CTX* ctx;
    unsigned char* rawOut = new unsigned char[48]; // ToDo Remove Hardcoded Value

    Info info = m_header.processKeyInfo();
    if (NULL == info.nonce)
        return NULL;

    if (!(ctx = EVP_CIPHER_CTX_new()))
        return NULL;

    if (!EVP_DecryptInit_ex(ctx, EVP_aes_256_gcm(), NULL, pszMasterKey, info.nonce))
        return NULL;

    if (!EVP_DecryptUpdate(ctx, NULL, &len, m_header.aad, m_header.aad_len))
        return NULL;

    if (!EVP_DecryptUpdate(ctx, rawOut, &len, m_header.encryptedValue, m_header.encryptedValueLen))
        return NULL;

    // Finalise the decryption. A positive return value indicates success,
    // anything else is a failure - the plain text is not trustworthy.
    ret = EVP_DecryptFinal_ex(ctx, rawOut, &len);

    ERR_print_errors_fp(stderr);

    EVP_CIPHER_CTX_free(ctx);

    if (ret > 0)
    {
        return rawOut;
    }
    else
    {
        return NULL;
    }
}