Singletons and ICommand - wpf

Question

Is there some special rules I have to be careful about when binding my view to the commands defined in my singleton ViewModel, opposed to normal (non-singleton) ViewModels?
All my ViewModels except the one in question behave normally. Each of them exposes two public members called HasChanges (bool property) and SaveChanges (method) that I'm calling in the CanExecute and Execute functions of my commands.
While all other Views behave normally, enabling/disabling the buttons when the value of HasChanges changes and saving the contents when those buttons are clicked, the only ViewModel that implements Singleton pattern happens to call CanExecute only upon the first loading of the View.
After that, any number of PropertyChanged events raised from within that ViewModel (all my ViewModels implement INotifyPropertyChanged) do not affect the disabled state of the button.
Wondering what I'm missing here.
Here's the singleton ModelView:
Public NotInheritable Class MyViewModel
Private Shared ReadOnly mInstance As New CommonListsViewModel
Public Shared ReadOnly Property Instance() As CommonListsViewModel
Get
Return mInstance
End Get
End Property
Public Property SaveChangesCommand As ICommand
Private Sub New()
SaveChangesCommand = New Commands.SaveChangesCommand()
End Sub
Public ReadOnly Property HasChanges As Boolean Implements IEditorViewModel.HasChanges
Get
...
End Get
End Property
Public Function SaveChanges() As Boolean Implements IEditorViewModel.SaveChanges
...
End Function
End Class
Here's the command:
Friend Class SaveChangesCommand
Inherits CommandBase
Public Overrides Function CanExecute(parameter As Object) As Boolean
Return MyViewModel.Instance.HasChanges
End Function
Public Overrides Sub Execute(parameter As Object)
MyViewModel.Instance.SaveChanges()
End Sub
End Class
And here's my View:
<Grid DataContext="{x:Static local:CommonListsViewModel.Instance}">
<Button Command="{Binding SaveChangesCommand}">
</Grid>

Answer 1

There shouldn't be any difference in the behavior due to the fact that a class follows the singleton pattern. Anything with a reference to the singleton will not care how the construction of the class is restricted, only that the instance of the class exists.
The problem probably lies in a lack of a proper link between the PropertyChanged event of the ViewModel and the CanExecuteChanged on the Command.