New tab in WPF - items.add from buttons vs methods - wpf

Question

OK, so I have a window called PictureWindow which displays pictures (I've cut out the code not related to making tabs). The TabControl is named "itemsTab". Using a button press, I can make a new tab no problem. But using the same operations inside a called method doesn't work. Using the buttonTab_Click method makes a new tab, the newTab method does not.
The only real difference I can see is due to the sender and RoutedEventArgs objects - how do these effect the operation here? Or is there something else I'm missing?
Thanks in advance.
Edit To make things even stranger, the newTab method does make a new tab, but only if it is called in the PictureWindow constructor method. If I have the following a new tab is made.
public PictureWindow(string current)
{
InitializeComponent();
newTab(current);
}
But if I call the method anywhere else it doesn't work.
public partial class PictureWindow : Window
{
public PictureWindow(string current)
{
InitializeComponent();
}
private void Window_Loaded(object sender, RoutedEventArgs e)
{
}
private void buttonClose_Click(object sender, RoutedEventArgs e)
{
this.Close();
}
private void buttonTab_Click(object sender, RoutedEventArgs e)
{
TabItem newTab = new TabItem();
newTab.Header = "New Tab!";
itemsTab.Items.Add(newTab);
}
public void newTab(string current)
{
TabItem newTab = new TabItem();
itemsTab.Items.Add(newTab);
}
}