I press the red button, buy a ticket, try to walk through/jump over the barrier and nothing happens. No error message. Meanwhile other people are walking in past me. This has been happening to me on and off for ages but now I can't get in at all.
I finally figured out what's wrong - if you use enter on 'buy a ticket' it doesn't work, if you click it with the mouse it does work. I was using enter key.