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AD7190: Data bits are not stable.

Category: Software
Product Number: AD7190
Software Version: Code Composer Studio
   I am using AD7190 ADC IC which is interface with MSP430FR6922 controller through SPI communication to read Load cell data.
My settings are as follows:-
1)  external crystal is used
2)  SINC4 (At higher output data rates (FS values less than 5), the sinc4 filter gives better performance than the sinc3 filter for rms noise and no missing code)
3) PARITY Disable
4) Single conversion disable
5) Notch off 
6) data rate 350 Hz
7) Chop off
8) External reference applied between REFIN1P and REFIN1N
9) Channel Ain1P and Ain1N selected
10) Burnout OFF
11) Ref detect disable.
12) Buffer disable
13) Bipolar mode
14) Gain 128 volatge range is ±39.06 mV

I have attached code for your reference purpose.When load cell is connected to AD7190 we get only MSB 12 bit stablefor 128 gain.After that I have changed the gain settings and observations are as follows:- gain mV fluctuations range
128 GAIN 0.016mV fluctuations in mV readings 64 gain 0.006mV 32 gain 0.004mV 16 gain 0.003mV 8 gain 0.001mV

after that, I power off the controller board and taken the readings after 5/6 hrs. I have not changed the supply and even wiring is same. I observed is for every gain settings we got 12 bit fluctuations in readings(0.01mV fluctuations). Formulae to calculate mV is:- Voltage(Ain)=(Code-2^23) Vref /(2^23*Gain)

Please help where I am getting wrong??