On the back side of 1Y2 rack I found a cable, CAB-1X2-LSC_7, which is supposed to be connected to the whitening filter was disconnected.
I plugged it back and confirmed that the whitening filter is under control of EPICS.
Now all the gain sliders seem to be working because I can change the amplitude of signals with the sliders.
To check if the gain sliders are working or not, I intentionally disconnected all the inputs to the whitening filter.
Then I brought a gain slider of interest to the maximum. Due to the big gain I was easily able to see noise lying above ADC noise.
Also if the gain slider is 0 dB, which is the minimum value, the spectrum becomes just ADC noise.
In this way I checked all the gain sliders from PD1 to PD4. The picture below is just an example screenshot when I was doing this test.
Note that each filer is designed to have two poles at 150 Hz and two zeros at 15 Hz.
While checking whitening filters on the LSC rack, I found some epics controls for the whitening looked not working.
So I powered two crates off : the top one and the bottom one on 1Y3 rack.
These crates contain c1iscaux and c1iscaux2. Then powered them on. But it didn't solve the issue.