You have the belt too loose. There may be other reasons as mentioned by others - bad fit, packed wrong, etc. - but the belt should be tightened so the load rides in the proper place. If the belt doesn't tighten any more than you can get it and it still slides, it's too big for you. And, also, there should be compression straps on the pack itself to snug up the load against the frame to keep the weight on your hips and not bouncing around below them.