If anyone having the same problem in Ubuntu 20.04 then he can use this trick that works for me.I have faced this problem when using Discord,Google Meet,Facebook video call.
I am using a software to change my camera settings named guvcview.
At first, sometimes my webcam becomes dark and I used to open guvcview and make some changes in webcam setting like toggle powerline frequencey
option and set it 50hz/60hz/disable and then the webcam becomes normal.
Then one day i installed Discord and now the darker problem is happening every time I use my webcam like google meet , Facebook video chat etc.
I have searched a lot and tried every solutions but didn't work for me.
Then I did some experiments and find a weird solution :
Before using webcam in any software I toggle the powerline frequencey
option and then open Facebook video call where the cam is not darker amd i is normal. Then I open Google meet and that's how it worked for me.
Though i couldn't find any permanent solution but it's working fine and you have to do it everytime you go for a google meet meeting.
I know my solution sounds weird but If you are facing this issue then have a try and if you find any better solution then please inform me.