Recently one of my colleagues came to me saying that there is a problem with the ValidationSummary. It comes first time the window is loaded, but when we open the same window next time, the ValidationSummary is not coming. He shared the link that someone was also facing the same problem and this is kind of issue with the ValidationSummary class.
MSDN Forum post (Quite old though):

I analyzed the post which was also complaining about the same problem. The user have already provided a POC to replicate the issue and I analyzed the problem what they're all doing the wrong. 

Problem area 1: They are keeping the child window instance as a field in the MainWindow class and assigning the dataContext in OpenChild window event of MainWindow.

Problem area 2: Resetting the datasource on closing of the ChildWindow.

Affected area: ValidationSummary get bound with the dataContext validation fields first time when the constructor of the ChildWindow is called. So first time it's working fine as per above scenario. Now when you close the window the dataSource is reset to null. Now the ValidationSummary have no bindings left with DataContext anymore. If you try to open the window again which is already initialized once and try to run the validation, there'll be no Validation summary as the bindings are gone with existing instance of ChildWindow.


Either don't use the ChildWindows object as field or Don't reset the data context. The validation summary will behave as it should.

Here's the updated sample of your application.