One of the most important steps to a good repair is surface prep. This photo shows a topping that was used to level a floor prior to final flooring application. The topping failed due to almost no surface prep. There is paint, dry wall, mud and dirt that inhibited a bond. The surface should have been washed and the mechanically abraded to help the repair mortar bond. This repair had a hollow sound and a section was demoed to investigate. You can see how the orange paint and other contaminates transferred to the underside of the mortar. The entire repair has to be removed and redone.