there is a method that you have to get use to of swirling and hovering the syphon just over the sand, in order to get at the debris, i never got use to that method and gave up on sand, i tried it twice in two different tanks, and loved the way it looked until it got dirty.
You also need to sift through the sand or get fish/snails that will do it for you else it will compact and anerobic bacteria will build up in pockets in the sand bed.