I would scar the surface with a chisel to give the grout something to hold. When one pours on top of a relatively smooth surface it takes alot of prep work. Try a technique involving a thin brushed-on layer of portland cement. As the portland gets tacky pour your floor leveling compound and prepare to trowel. P.S Theses grouts have good compressive strength and a piece of diamond mesh will hold it together verses no reinforcing.