You can try to switch different channel and you can try to see if channel selection is happening correctly or not.
What I meant by kickback, if channel is selected then at sample start and sample end time instance, you should see a kickback on the analog input. It should be visible in oscilloscope. Only selected channel should show this and other unselected channel should not show this. By observing this you can makeout whether channel selection is happening correctly.
If everything is correct then only thing comes to my mind is that you can try with another device. (cause during all these debug if the device is damaged somehow.)
On SDO line first 4 bits are channel address and then data bit comes out. Looking at channel address also you can make out whether channel selection is correct or not.