How do I get a lower berth?

How tf should I get a lower berth while booking a seat in IR, this is not a necessity, but I would love a lower berth seat (window seat which means), since UB/MB and non window seats SUCK, I am not the kind of guy would just use laptop or play with my phone or sleep the whole travel, especially 24 hours travel.

I am a 20yo male, I tried booking a seat on New Delhi to Bhubaneswar Rajdhani 3AC, there were 300 fucking empty seats, I had my preference side lower (since I feel choosing lower might give me middle berth, which is even worse), still I got an Upper berth, not even side upper (which is better, since it is a bit low and quite easy to climb and get down and go to the door to have a nice look outside (why I desire window seat), also, if I have a side upper seat, I might get to sit on the side lower (window seat) during when the side lower passenger is not lying down.

But why UB? There were 300 empty seats and I do not believe that there were no side lower berths for me, or even lower.

Also, recently I booked a seat on Purushottam express (from Bhubaneswar to new delhi), on 2AC, I choosed side lower, and got a side lower, no problem, One more time I did this last year and I got a side upper (no issues to me). Seats available were like 12-13. Then why the fuck this issue with Rajdhani?

Please suggest me some tips and tricks to get a lower berth or side lower or even side upper berth.

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