Mockito when and thenReturn statement not working for 2 method calls in Java

Inside my Test class, I have the following mocking statement:

when(metadata.getGranularity(message)).thenReturn(new Assembly.Partition.Builder.build());

Basically, I am calling this above statement through two different test methods. One is existing and works fine, second is my newly written code which calls the same method. It's mentioned inside the setup method. It gets executed in both the cases and when I evaluate the value, it gives an object reference in both the cases, like this:

result= {Assembly$Partition@3793}

The code in my class that it is mocking is:

Assembly.Partition granularity = metadata.getGranularity(message);

But when the debugger goes from test method to the code, the builder creates an object reference in the first case i.e. granularity= {Assembly$Partition@3892}, but in the second case, it is giving the reference as null.

Also, sometimes while debugging, it gives me this debug error that Partition cannot be returned by toString().

Edit

Existing test method is this:-

public void publish()
filePublisher.publishFirst(message, event, name);
verify(file publisher, times (1)).publishFile(anyString(), anyList(Mylist.class));

And my new method is:-

public void publish2()
filePublisher.publishSecond(date, id, type);
verify(file publisher, times (1)).publishFile(anyString(), anyList(Mylist.class));

Both methods compute various data to call the publishFile method.

You really have not added enough (real) code to pin this down so don't expect a real answer! Here's a guess:

when(metadata.getGranularity(message))... 

... only mocks when that exact message arrives. The publish2 example is

filePublisher.publishSecond(date, id, type);

where date != message.

Try this:

when(metadata.getGranularity(any())).thenReturn(new Assembly.Partition.Builder.build());