Irctc — seat allotment

Address:

Chennai, Tamil Nadu

I have booked a ticket on[protected] for train no:16853 PNR no:[protected], there was 215 seats available at the time, I have selected "Book ticket if 2 lower berth allotted", but system demanding me as "Conditions not met plz click YES to book a ticket anyway or NO to cancel", PLEASE justify my question, how it is possible that there is no availability of 2 lower birth seats in same compartment when there is 215 seats available in the whole train? .It happens many times for me, I'm loosing my hope on online booking system.