You may be unable to see the book button due to the following reasons:-
· You will be unable to book the train between 2345 hours to 0030 hours because the indian railway system doesn't allow to book the train between this period.
· It may show 'check availability' enquiry has failed. It may happen due to miscommunication between Goibibo and IRCTC system.
· If all the 'waitlist' quota has been filled, you won't be able to book the train.