My Menu/Navigation Bar disappears as I Scroll up my pge

My Menu bar slides up and goes away as I scroll up my page…How do I make it so that it stays static in place on every page that I build or on just a single “One Page” responsive website style? I think I have my pages set to full width, but I’m not sure if my home page settings or any other page settings would have anything to do with why my menu header slides up and just disappears. Seems to me that the menu header would have an automatic default to stay as static in place regardless of size and style features of any of my pages. Please help me resolve this if you can…I am new to website building and WordPress, although I’ve done fairly well so far, but this has me stumped and I’ve been trying to fix it all day. Attached are 3 screen shots, showing the progression of my menu/navigation bar disappearing as I scroll down my page.

Thank You,
~Wendy