Please note that you cannot set ADC buffer gain to 2x, 4x, 8x for all types of voltage ranges that the ADC will support. Please find the attached snip which is taken from Delta sigma ADC component datsheet.
If you are following it and still you are facing issues means please come back.
adcrange.PNG 71.2 K
Thank you for your reply.
Check again, paying attention to the effective range.