Android Listview not displaying added items - android-layout

Question

I had a listview that was working fine and displaying Items which we're being created after taking a photo and then added. Now, for some reason, it doesn't.
What's particulary strange is that if I add the items in the same place where I initially create and set the adapter, the phone DOES show them. But when I do so in a later stage, it doesn't!
EDIT - After some more trial and error, it looks as though the database related commands are problematic! See commented below.
RELEVANT JAVA CODE
Cart _cart;
private AppDataBase _db;
#Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_cart_review);
init(getIntent().getStringExtra("calling activity"));
}
private void init(String callingActivity)
{
_cart = new Cart();
_pricingServices = new PricingServices(this);
ArrayAdapter<Item> adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, _cart.items);
ListView itemListView = findViewById(R.id._dynamic_item_list);
itemListView.setAdapter(adapter);
_cart.items.add(new Item(5, "blame", 4)); //these work!!!!
_cart.items.add(new Item(5, "bloom", 4)); //!!!!!!
_db = Room.databaseBuilder(getApplicationContext(), AppDataBase.class, "class").fallbackToDestructiveMigration().allowMainThreadQueries().build();
if(callingActivity.equals(MainActivity.class.getSimpleName()))
{
_cart = new Cart(); //When these are commented out
_cart.setId((int)(_db.cartDao().insert(_cart))); //it seems to work
}
else
_cart.items.addAll(_db.itemDao().getItemsByCartId(getIntent().getIntExtra("cart id", -1)));
}
public void OnItemDescriptionDone(String description)
{
_description = description;
Item item = new Item(_convertedPrice, _description, _cart.getId());
_cart.items.add(item); //This doesn't update the UI !!!
}
And here is the layout of the activity
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".CartReviewActivity">
<ListView
android:id="#+id/_dynamic_item_list"
android:layout_width="0dp"
android:layout_height="0dp"
android:layout_marginBottom="24dp"
android:layout_marginTop="24dp"
app:layout_constraintBottom_toTopOf="#+id/button_capture_image"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="#+id/textview_cart_name" />
<ImageButton
android:id="#+id/button_capture_image"
android:layout_width="wrap_content"
android:layout_height="55dp"
android:layout_marginBottom="16dp"
android:onClick="OnCameraButtonClick"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:srcCompat="#drawable/camera_icon" />
<TextView
android:id="#+id/textview_cart_name"
android:layout_width="0dp"
android:layout_height="31dp"
android:layout_marginEnd="24dp"
android:layout_marginLeft="24dp"
android:layout_marginRight="24dp"
android:layout_marginStart="24dp"
android:layout_marginTop="16dp"
android:text="The Cart name will be"
android:textColor="#android:color/darker_gray"
android:textSize="24sp"
android:textStyle="bold"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>