Update Class values that have Bindig to TextBox in WPF

How about, I found a problem, I have textbox that have binding , but I realized that the value in the class is not updated until I click on the textbox , I mean that I shows the desired value but if I do not click on the textbox when I send to save that property it appears as Null , how can I solve this? Since those textbox I want to hide, but still keep the values. I am using MVVM and WPF

Thanks for your help. Greetings.