Botton menu

Hello everyone I just put a button on the menu, but behaves strangely, has no class, just put it but at the time a preview and pass the hand on the button disappears as if it had an effect over!!

I have tried placing a class although in the design mode it is perfect, it continues doing the same thing when passing the muose on the button, it disappears! someone knows how to fix that type of mishap, since I want a fixed button always