可能的原因是API的调用存在问题，可以尝试检查API调用时所使用的HTTP方法、URL、Headers和Body是否正确。以下是Java代码示例，演示如何通过 HttpURLConnection 进行调用：

import java.net.HttpURLConnection;
import java.net.URL;
import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.Base64;

public class ChallengeQuestionAPI {

    public static void main(String[] args) {
        String url = "https://api.example.com/v6/challenge-questions";
        String username = "your_username";
        String password = "your_password";

        try {
            URL obj = new URL(url);
            HttpURLConnection con = (HttpURLConnection) obj.openConnection();
            con.setRequestMethod("GET");

            String encodedAuth = Base64.getEncoder().encodeToString((username + ":" + password).getBytes());
            con.setRequestProperty("Authorization", "Basic " + encodedAuth);

            int responseCode = con.getResponseCode();

            if (responseCode == HttpURLConnection.HTTP_OK) {
                InputStream inputStream = con.getInputStream();
                BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
                String inputLine;
                StringBuffer response = new StringBuffer();

                while ((inputLine = reader.readLine()) != null) {
                    response.append(inputLine);
                }
                reader.close();

                String responseData = response.toString();
                System.out.println(responseData);
            } else {
                System.out.println("GET request not worked. ResponseCode: " + responseCode);
            }

        } catch (Exception e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

上述代码会在控制台输出API响应数据。如果仍然无法工作，请联系API提供商以获取更多支持。