What roots? There's your problem - for some reason, your cactus has no roots, either because of too much or too little water. A plant with no roots can't absorb moisture.
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and proclaiming...."WOW What a Ride!!" -Mark Frost
President: Orchid Society of Northern Nevada
Webmaster: osnnv.org