Unsure why 2 way binding is not working correctly.

I have a ObservableCollection of Category which has a selected property and I'm dynamically creating a series of checkboxes which I want to two way bind.

I populate the checkboxes correctly, and when I populate the form these will appear correctly bound. However I want to be able to programatically set the checkboxes and was assuming I could change the state of the item in the collection and the checkbox would reflect this but that was not the case. I could do it the other way around and programtically determine the control was a checkbox and then set its state but that seemed more complicated and did not seem to reflect what I thought should be happening with 2 way binding. Any ideas what I'm missing on setting up the binding.

public partial class CategorySelect : ContentPage
{
    public ObservableCollection<Category> SelectedCategories { get; set; }

    public CategorySelect(ObservableCollection<Category> list)
    {
        InitializeComponent();

        SelectedCategories = list;

        //Create a list of check boxes for each category and bind to the collection
        int index = 0;
        foreach (var item in SelectedCategories)
        {
            var b = new Syncfusion.XForms.Buttons.SfCheckBox() { Text = item.Description };

            b.BindingContext = SelectedCategories[index];
            b.SetBinding(Syncfusion.XForms.Buttons.SfCheckBox.IsCheckedProperty, "Selected", mode: BindingMode.TwoWay);

            SelectionList.Children.Add(b);
            index++;
        }


        private void Button2_Clicked(object sender, EventArgs e)
        {
            foreach (var item in SelectedCategories)
            {
                item.Selected = true;

            }
        }

        private void Button_Clicked(object sender, EventArgs e)
        {
            // Should be able to better do this with 2 way binding between the collection and the 
            // controls that are added - meaning we should just be able to modify the collection.
            foreach (var item in SelectionList.Children)
            {
                if (item.GetType() == typeof(Syncfusion.XForms.Buttons.SfCheckBox))
                {
                    ((Syncfusion.XForms.Buttons.SfCheckBox)item).IsChecked = true;
                }

            }
        }

}