In order to display properly in landscape mode the tablet needs to provide a minimum of 650dp (display pixels) screen height when in landscape orientation. Due to the nature of the Android platform and the variability between devices we have found that some large Android tablets scale the display in such a way that the minimum number of pixels is not available, and this forces the app into portrait mode.
dCS Mosaic Control bases its mode determination on what is reported by the Android OS. This value is reported in Mosaic Control under Settings > Support > About dCS Mosaic . If the smaller of the two values listed under detected screen size is less than 650dp then you will need to investigate whether or not the scaling setting on your tablet can be modified.