ComboBox in Windows Phone 8.1

Hello , I use Vb2013 Ultimate with emulator Windows Phone 8.1 . If the Items of my ComboBox ( < = 3 items), when you click on it, covering TextBox cannot be selected , because they appear as disabled. if ComboBox is flyout windows, after selecting an item, you don't see it in the box of the ComboBox. I see it only if I click over.
Greetings