Error consuming web service POST urlencoded with Android Studio retrofit2

I'm just trying to consume services with Android Studio and GET services have worked well, but with POST services it does not work for me. I have tried several ways that I have found out there and the answer always goes by the onFailure side.

In my interface I have the following:

public interface APIService {

@FormUrlEncoded
@POST("nombreServicio")
@Headers("Content-Type: application/x-www-form-urlencoded")
Call<RespuestaPost> postRegistrar(
        @Field("param1") String param1,
        @Field("param2") String param2,
        @Field("param3") String param3); }

The ResponsePost class with their respective set and get:

public class RespuestaPost {
    private String mensaje, respuesta, data;
    private int estado;
    //AQUÍ TODOS LOS MÉTODOS SET Y GET
}

The retrofit client that has worked for GET services:

public class RetrofitClient {
    public static final String BASE_URL = "https://192.168.1.xx:xxxx/serviciosExternos/api/v1/app/";
    public static APIService RETROFIT_CLIENT;

    public static APIService getInstance() {
        //if REST_CLIENT is null then set-up again.
        if (RETROFIT_CLIENT == null) {
            setupRestClient();
        }
        return RETROFIT_CLIENT;
    }

    private static void setupRestClient() {
        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .build();
        RETROFIT_CLIENT = retrofit.create(APIService.class);
    }
}

And finally in my MainActivity:

public class MainActivity extends AppCompatActivity {
 private static final String TAG = "CONSOLA: ";
 TextInputEditText etParam1, etParam2, etParam3;
 Button btn1;
 private APIService mAPIService;
 Call<RespuestaPost> call;

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mAPIService = RetrofitClient.getInstance();

        etParam1 = findViewById(R.id.Param1);
        etParam2 = findViewById(R.id.Param2);
        etParam3 = findViewById(R.id.Param3);

        btn1 = findViewById(R.id.Btn1);
        btn1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String p1= etParam1.getText().toString().trim();
                String p2= etParam2.getText().toString().trim();
                String p3= etParam3.getText().toString().trim();
                if(!TextUtils.isEmpty(p1) && !TextUtils.isEmpty(p2) && !TextUtils.isEmpty(p3)) {
                    sendPost(p1, p2, p3);
                }else{
                    //INCOMPLETO
                }
            }
        });



    }

    private void sendPost(String p1, String p2, String p3) {
        call = mAPIService.postRegistrar(p1, p2, p3);
        call.enqueue(new Callback<RespuestaPost>() {
            @Override
            public void onResponse(Call<RespuestaPost> call, Response<RespuestaPost> response) {
                Log.i(TAG, "Se envió exitoso.");
                if (response.isSuccessful()) {
                    // tasks available
                    String respuesta = response.body().toString();
                    Log.i(TAG, "Respuesta: " + respuesta);
                } else {
                    // error response, no access to resource?
                    Log.i(TAG, "1. Error response");
                }
            }

            @Override
            public void onFailure(Call<RespuestaPost> call, Throwable t) {
                Log.e(TAG, "2. Error no se envió.");
            }
        });
  }
}

With GET services it gives me the normal answer, but with the POST services it always throws the onFailure response: "2. Error was not sent.". I appreciate if someone could tell me if I'm doing something wrong in the process, since the services work correctly when consumed from other places like Postman.

Edit: After putting in Log.e(TAG, "2. Error no se envió. "+t.getMessage()); I got the following error:

  

java.security.cert.CertPathValidatorException: Trust anchor for   certification path not found.

Edit: The error was because of the SSL certificate. In my case the solution given by Retrofit can be found in the following link: Retrofit 2 - How to trust unsafe SSL certificates (self-signed , expired)