If your booking is failing continuously, you can find the reason on the booking summary page by going to "My Orders", locating the booking and opening the booking summary.
In some cases, the booking failure would be because certain berth preferences chosen by you at the time of booking such as “Book only if at least 1 lower berth is provided” etc. which are not met. In such cases, you can retry and make a successful booking by removing the berth preference.
In few other cases, it is possible that there are genuine IRCTC failures due to reasons like the seats being filled, bookings no longer accepted etc. In such cases, you would not be able to get a successful booking.
In majority of cases, the failures would have been due to connection issues on IRCTC. In such cases, we suggest you to try after sometime.