There could be a number of reasons for this: The full experience and Star Tours only operate on select weekends and bank holidays due to the demands of the production.
We release tickets periodically throughout the year, so you may also be looking too far ahead where tickets have not been released yet. We strongly recommend you sign up to our email alerts so we can drop you an email to let you know once new dates are released. It may also be that the timeslot you’re looking to book has already sold out.