I recently built a rig for a friend of mine and discovered that for some reason the system doesn't recognise any of the ram apart from the stick occupying the first slot... At first we thought it may have just been a bad board so we got a replacement to no avail. Any help would be appreciated.
You may find a "ganged" and "unganged" setting in the bios; try changing that (enabled or disabled). Some boards use a "memory mapping" setting instead, which can also be changed. Are you mixing brands of ram or using different speeds?