Checkbox value in wpf treeview is not equal to value of bound property

I have a treeview in a wpf form that initially binds correctly but certain combinations of checking and unchecking items in the tree cause the binding to fail to hold.

Here is the property;

        const string HAS_ACCESS = "HasAccess";

        public bool? HasAccess
    {
        get
        {
            return hasAccess;
        }
        set
        {
            if (hasAccess == value) return;
            if(!AllowUpdate(value)) return;
            //Debug.WriteLine("{0}  tried to set hasAccess to a value of {1} D = {2}, A = {3}, u = {4}, AllowChange = {5}, I = {6}", myPermission.Name, value, descendantChanged, ancestorChanged, userInstigatedChange, AllowUpdate(value), isInitialising);
            ResetChangeBools();
            hasAccess = value;
            Debug.WriteLine("HasAccess = {0}", HasAccess);
            isInitialising = false;
            RaisePropertyChanged(HAS_ACCESS);
        }
    }

This is the xaml;

                                            <CheckBox Grid.Column="0" Grid.Row="1"
                                          IsThreeState="true"
                                          Content="{Binding MyPermission.Name}"
                                          Click="CheckBox_Clicked"
                                          IsChecked="{Binding HasAccess, Mode=OneWay}"/>

I must stress that the correct value for HasAccess is displayed when the treeview is initialised and does update when changed MOST OF THE TIME but if I check a child item in the treeview (that correctly checks its parent), uncheck its parent (which correctly unchecks its children) then recheck the initial child element the parent no longer checks. I included the Debug.writeline to check that the values were being correctly set and although I can see the correct values in the Output window they are not shown in the checkbox. Ahy help would be greatly appreciated.