Like bc suggested, tear out and remove the old stuff and clean concrete substrate throughly.
Some Tips Very smooth concrete substrates should be etched beforehand (improves bonding surface) using an acid designed for this purpose. Follow manufacturer's product recommendations.
Mix leveling powder with a 50/50 solution of clean water and acrylic latex admixture. The acrylic latex additive will greatly enhance the levelers mechanical bond.
Prior to installing leveling mortar, pre-mist concrete substrate with clean water and lightly dust the floor with dry leveling powder. Use a broom to spread the slurry on all surfaces to be covered with leveler. Also improves bond.
Install leveler according to manufacturer's instructions. Each batch of floor leveler should be mixed the same.
Note: Portland cement floor levelers are not designed to be a stand alone floor covering because they do not wear well. They should be covered with the proper flooring as soon as the leveler has properly cured.