Describe how zinc oxide is made from zinc sulfide

CO2 adsorbed onto the ZnO, which is progressively lost during the experiment ? Are you sure about the quality of your ZnO ? If it is not a fresh one, it might be carbonated, and decompose into ZnO + CO2, thus losing weight. Alternatively, you can check if you produced ZnS through the expected reaction ZnO + H2S → ZnS + H2O : Take a small part of your sorbent on a lab tile, add a few drops of H2SO4 (4 mol/L), observe and smell. If ZnS is present, then a little amount of H2S gas will be produced: you will see it or at least smell it, as the nose is sensitive to 3 ppm of H2S in air. Then you'll conclude whether the expected reaction occurred or not.