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.