Enable button based on TextBox value (WPF)

This is MVVM application. There is a window and related view model class.

There is TextBox, Button and ListBox on form. Button is bound to DelegateCommand that has CanExecute function. Idea is that user enters some data in text box, presses button and data is appended to list box.

I would like to enable command (and button) when user enters correct data in TextBox. Things work like this now:

• CanExecute() method contains code that checks if data in property bound to text box is correct.
• Text box is bound to property in view model
• UpdateSourceTrigger is set to PropertyChanged and property in view model is updated after each key user presses.

Problem is that CanExecute() does not fire when user enters data in text box. It doesn't fire even when text box lose focus.

How could I make this work?

Edit:
Re Yanko's comment:
Delegate command is implemented in MVVM toolkit template and when you create new MVVM project, there is Delegate command in solution. As much as I saw in Prism videos this should be the same class (or at least very similar).

Here is XAML snippet:

    ...
    <UserControl.Resources>
      <views:CommandReference x:Key="AddObjectCommandReference" 
                              Command="{Binding AddObjectCommand}" />
   </UserControl.Resources>

   ...
   <TextBox Text="{Binding ObjectName, UpdateSourceTrigger=PropertyChanged}"> </TextBox>
   <Button Command="{StaticResource AddObjectCommandReference}">Add</Button>
   ...

View model:

   // Property bound to textbox
   public string ObjectName
    {
        get { return objectName; }
        set { 
            objectName = value;