Greetings aspirant!

Exam centres  are alloted according to their availability. Preferences are given for the purpose of your comfort, if first preference isn't available for some reason then the candidate is alloted the second preference. Usually students get first preference but it can happen that due to large number of students selecting the same centre as their first preference seats gets filled quickly. Their is not much you can do if you have got your second preference.

Hope it helps.
Thankyou!