Could be many reasons. Mine go through stages of hiding and general wandering around the Viv and Looking for escape routes.
Is there anything you've changed recently? Sometimes small insignificant things can make them do this for example: I used to use the top of my old viv as a monitor stand for my PC, yet when I moved the monitor about 2 inches backwards my snakes went odd. I later found out that the cooling fan on my monitor was blowing air straight into a tank vent, and they obviously didn't like the smell/taste of it.
I've also known a snake to start hiding all the time, and being jumpy if anyone touched it whenever a glass was put near one side of his viv.
Other times he could just be deciding to be a lazy snake. Have a look and see if you think anything has changed recently, New TV?? anything like that. Have you handled anything that could conceivably be a predator to Jeff? Lots of possibilities. When was he last fed? and what end of the viv is he hiding in mostly? the colder or warmer end?