The ticket issuing machine at Bat & Ball station frequently does not issue off-peak tickets until about 09.15 hours. With the first off-peak departures at 09.21 to Sevenoaks and 09.25 to Blackfriars this can cause problems when there are many people waiting to purchase tickets for these services.
This “feature” is also implemented at Sevenoaks. Southeastern say that the time is set so that people cannot accidentally buy an off-peak ticket that would not be valid on the next train. But it also means that people may well accidentally buy a full-price ticket if they arrive early for an off-peak train – and of course Southeastern don’t complain about that. (It would be better for the ticket machine to give a warning “this ticket is not valid until 0921” rather than refuse to offer it at all.)
However, there is a way round this. If you select the option to buy a ticket for travel on a future date, when prompted to enter the day on which you want to travel enter the current date then tickets at off-peak fares are available.
To avoid this problem, I usually buy my tickets online in advance and then I only need to collect them from the ticket machine at any time.