WPF Tooltip binding not updating

Good afternoon all,
I have to work with a legacy Winforms application but I'd like to start migrating it to WPF. It doesn't have a tooltip control now so I'd like to use a WPF tooltip object.

I create a single global instance of a tooltip object. I've bound the controls within it and my application sets the datacontext of the tooltip. I can manually show and hide the tooltip just fine. The first time I hover over an object it picks up the bound data perfectly and works great. When I move over another control the tooltip datacontext is changed but the displayed data is never reloaded.

I tried implementing a property changed event and use the INotifyPropertyChanged interface in the object I bind to. It appears the wpf tooltip is not listening to the event.

I tried setting the binding mode to Oneway (it's a display only tooltip).

The tooltip is created programmatically:

// build the tooltip window.
System.Windows.Controls.Image img = new System.Windows.Controls.Image();
img.Width = 50;
img.Height = 60;

// bind the image
System.Windows.Data.Binding imageBinding = new System.Windows.Data.Binding("PatientImage.Data");
imageBinding.Mode = System.Windows.Data.BindingMode.OneWay;
imageBinding.Source = bed;
img.SetBinding(System.Windows.Controls.Image.SourceProperty, imageBinding);

// wrap image in a border
System.Windows.Controls.Border brdr = new System.Windows.Controls.Border();
brdr.BorderBrush = System.Windows.Media.Brushes.Blue;
brdr.Margin = new System.Windows.Thickness(6);
brdr.Child = img;

System.Windows.Controls.WrapPanel wp = new System.Windows.Controls.WrapPanel();

System.Windows.Controls.TextBlock tb = new System.Windows.Controls.TextBlock();
tb.Background = System.Windows.Media.Brushes.LightBlue;
tb.Foreground = System.Windows.Media.Brushes.Blue;

// bind the text block
System.Windows.Data.Binding textBlockBinding = new System.Windows.Data.Binding("TooltipText");
textBlockBinding.Mode = System.Windows.Data.BindingMode.OneWay;
textBlockBinding.Source = bed;
tb.SetBinding(System.Windows.Controls.TextBlock.TextProperty, textBlockBinding);

wp.Children.Add(brdr);
wp.Children.Add(tb);

bedTooltip = new System.Windows.Controls.ToolTip();
bedTooltip.Content = wp;

Any idea why this doesn't work? Maybe I need to use a tooltip object for each control instead of a single global one as a workaround?