How to update Progress from state of ObservableCollection<T>?

Question

I have an ObservableCollection<T>, T object has nested ObservableCollection<U>, U class has a IsCompleted boolean. How can I collect on my ViewModel the Progress which is the % of completed.

TestPage:

<ProgressBar Progress="{Binding Progress}" />

class TestViewModel
{
    [ObservableProperty]
    public ObservableCollection<T> _items;
    public double Progress
    {
        get
        {
            return ((double)Items.NestedItems.SelectMany(e => e.NestedItems).Count(s => s.IsCompleted) / (double)Items.Sum(e => e.NestedItems.Count));
        }
    } 

    void Init()
    {
        // load data
    }  
}

class T
{
    public ObservableCollection<U> NestedItems { get; set; } = new ObservableCollection<U>();

}

class U :ObservableObjectBase
{
    bool IsCompleted {get;set;}
}

So with Progress I am trying to collect the percentage of IsCompleted of the nested item. The problem is Progress is not updated at all.