Why will my Dock not stay visible

I have always had my settings set so that the Dock is always visible. But in the last few months it keeps disappearing. When I checked the settings, it is still set NOT to automatically hide/show. I even followed the online instructions to us the Terminal to both Restart the Dock (didn't help), then. Reset Dock .. and it hasn't helped either. This is very annoying. Is there some way to get it to work properly?