Why Consturct not called sometimes

I have two userwidet,first one have a verticalbox and a canvaspanel,second one has a button.I add first to viewport and create a second add to verticalbox.The first one listen to action when F is release,add the second added before to the canvaspanel,it call the Construct.But when i make the second’s button do the same thing,it doesn’t call the Construct.