WPF-Prism可以执行不被调用的方法

I am coding a simple login UserControl with two TextBoxes (Username and Password) and a Login button. I want the Login button to be enabled only when the username and password fields are filled in. I am using Prism and MVVM. The LoginViewModel contains a property called LoginCommand that is bound to the Login button. I have a CanLoginExecute() method in my ViewModel but it fires only when the application comes up and then never again. So the Login button is never enabled. What am I missing?I am coding a simple login UserControl with two