Mocked method is never getting called

I'm trying to write a test case for a java class. I've mocked a method using Mockito, but the mock is never being used. Why?

Below is the structure of my java class:

class A {

 @Autowired
 private ClassB classB;

 publiv void methodOne() {
  methodTwo();
 }

 private void methodTwo() {
  ...
  int returnedValue = classB.someMethod();
  ...
 }
}

The test class is given below:

class ATest {

 @Mock
 private ClassB classB;

 @InjectMocks
 @Autowired
 ClassA classA;

 @Before
 public void setupTest() {
  MockitoAnnotations.initMocks(this);
 }

 @Test(expected = SomeException.class)
 public void testMethodOne() {
  when(classB.someMethod()).thenReturn(29);
  classA.methodOne();
 }
}

I've gone through the existing questions but have been unable to find an answer. If there is any question/answer that may help me, please point me to the same.

Thanks in advance!