You did not add enough zinc
In order for the cementation process to work the "free acid" that is capable of dissolving metal needs to be used up before the metal higher on the reactive series of metals can do it's job of cementing out the metal(s) lower on the reactive series of metals
Cementation is a redox reaction wherein metals higher on the reactive series of metals are oxidized (dissolved) thereby allowing the metals lower on the reactive series of metals to be reduced
But - that redox (or ion exchange) will not take place until the "free acid" - capable of oxidizing (dissolving) the lower reactive metal is "loaded" with those lower reactive metal ions
here is the reactive series of metals ------------
https://en.wikipedia.org/wiki/Reactivity_series
Kurt