How to book ferry from Split to Hvar town

I don’t seem to be able to book a foot passenger place on the 9608 route from Split to Hvar town. I see that this route has a ferry that departs split at 17.00 and also Route 9604 that departs Split at 15.00 but I do not seem to be able to book these ferries via the jadrolinija website. I wish to travel 31st July one way. Please can someone tell me how to do this? Thanks