How to test complex RxJava Observables

I am using RxJava in my android project and I'm actually starting unit testing my code.

I have following Observable method which I'm going to test

private Observable<Boolean> changeMyString (@NonNull String testString) {
    return OneClass
        .doSth1(testString)
        .subscribeOn(Schedulers.io())
        .observeOn(AndroidSchedulers.mainThread())
        .flatMap(testString2 -> Observable.create(
            subscriber -> {
                manager.setTestString2(testString2);
                AnotherClass
                    .doSth2(testString2)
                    .subscribe(
                        testString3 -> {
                            manager.setTestString3(testString3);
                            subscriber.onNext(success);
                        },
                        subscriber::onError,
                        subscriber::onCompleted
                    );
            }
        ));

My Test case looks like the following:

TestSubscriber<Boolean> testSubscriber = new TestSubscriber<>();
ClassToTest.changeMyString("TESTSTRING").subscribe(testSubscriber);

When I debug my Code it's not reaching this part here:

...
.flatMap(testString2 -> Observable.create(
        subscriber -> {
            manager.setTestString2(testString2);
            AnotherClass
                .doSth2(testString2)
                .subscribe(
                    testString3 -> {
                        manager.setTestString3(testString3);
                        subscriber.onNext(success);
                    },
                    subscriber::onError,
                    subscriber::onCompleted
                );
        }
    ));

How can I write a Unit test for this case? I can't figure it out, why it's not working...

Thanks for your help guys!

Alban