Why is the collect of a flow in a nested fragment (ViewModel) not called?

I have a setup where an Activity holds two fragments (A, B), and B has a ViewPager with 3 Fragments (B1, B2, B3)

In the activity (ViewModel) I observe a model (Model) from Room, and publish the results to a local shared flow.

val mSharedFlow = MutableSharedFlow<Model>` that I publish the model updates to: 
...
viewModelScope.launch { repo.observeModel().collect(sharedFlow) }

Fragments A and B (ViewModels) have access to the sharedFlow through (Parent) fun getModelFlow(): Flow<Model> There are no problems running the collects for each Fragment:

viewModelScope.launch { 
  parent.getModelFlow().collect { model -> doStuff(model) }
}

But, the problem is in the nested fragments (B1 etc.) In the fragment (ViewModel) for B1 I have another parent.getModelFlow() that in turn calls Fragment B (ViewModel) parent.getParentFlow()

I have no problem acquiring the flow (i.e the SharedFlow (as Flow from the activity ViewModel)); But the collect in B1 does nothing.

• Why am I not able to collect from the shared flow in the nested B1? (When A and B works fine)
• Am I already not taking some flow rules into consideration? Additional launch{}'es, other withContexts(Some?), flowOn, launchIn etc.?

(The providing of the flow is not the problem. Even if I create intermediary flows, or place the sharedFlow in a kotlin Singleton object I still have the same problem)

=== EDIT ===

I was asked to add more information, unfortunately (for all) I can't paste the actual code because it would just appear verbose and foreign (see my comment below). But here's some psuedo-code that should be equivalent.

One clarification, that you can see below, Activity, FragmentA, FragmentB, FragmentB1 (etc.) are all running at the same time- but only one of A and B is visible at one time.

class TheActivity {
    fun onCreate() {
        setupFragments()
    }

    /** Two fragments active at the same time,
    but only one FrameLayout is visible at one time */
    private fun setupFragments() {
        val a = FragmentA.newInstance()
        val b = FragmentB.newInstance()

        supportFragmentManager.commit {
            add(R.id.fragment_holder_a, a)
            add(R.id.fragment_holder_b, b)
        }
    }
}

class ActivityViewModel {
    val activityModelFlow = MutableSharedFlow<Model>()

    fun start() {
        viewModelScope.launch {
            getRoomFlow(id).collect(activityModelFlow)
        }
    }
}

class FragmentA { // Not very interesting
    val viewModel: ViewModelA
}

class ViewModelA {
    fun start() {
        viewModelScope.launch {
            parentViewModel.activityModelFlow.collect { model ->
                log("A model: $model")
            }
        }
    }
}

class FragmentB {
    val viewModel: ViewModelB
    val viewPagerAdapter = object : PagesAdapter.Pages {
        override val count: Int = 1
        override fun title(position: Int): String = "B1"
        override fun render(position: Int): Fragment = FragmentB1.newInstance()
    }
}

class ViewModelB {
    val bModelFlow: Flow<Model> get() = parentViewModel.activityModelFlow
    fun start() {
        viewModelScope.launch {
            parentViewModel.activityModelFlow.collect { model ->
                log("B model: $model")
            }
        }
    }
}

class Fragment B1 {
    val viewModel: ViewModelB1
}

class ViewModelB1 {
    fun start() {
        viewModelScope.launch {
            // in essence: 
            // - ViewModelB.bModelFlow -> 
            // - ActivityViewModel.modelFlow
            parentViewModel.bModelFlow.collect { model ->
                log("B1 model: $model")
            }
        }
    }
}

So, all of the connections of acquiring parentViewModels, DI, fragment creation etc. is all working fine. But B1 model: $model is never called! Why?