VS 2015 Popup does not display the second time without a message box

I have a form that starts with a popup. There are 2 buttons on the popup TRUE or FALSE.
When the button is pressed, the main form is displayed. it checks something and the popup is supposed to get displayed again.
When I click on the main form the popup does not get displayed unless I have a messagebox. I can't click on the TRUE or FALSE to continue.
How can I display the popup the second time without a messagebox and be able to click on the TRUE or FALSE button to continue?

Thanks in advance