NDVI satellite image

Hi, I am trying to calculate NDVI from an area containing 26k plots of land. With ArcGIS I do a FLOAT and a DIVIDE, then I apply zonal statistics by plot. The dates are from October 2019 to October 2020. I get very bad plot plots of land. I have to say that in this area two satellite images are superimposed, could it be the cause of my results? I don’t know what error I have…May be with the WCS connection…