Which train? Also, It's called as Lwer berth quota. When someone books a ticket on lower berth quota and cancels it later. It's not allocated/released instantly. It gets released only once the chart prepares.
So in your case, All the lower berth quota is exhausted and that's the reason you are not able to see any berths. You can take a chance of booking it and wait for the chart to get prepared to get the lower berth allotted. Hope this helps.