In reply to trish1968:
I was out the same time as you (we may have met at Symplegades) and heard about this from the owner of our accommodation, a local. For some reason the ferry company cancelled one of it's late sailings which caused the problems but after much complaining from the locals they changed the timetables back with an extra sailing and said they were sorry! Such is Greek public transport I think. They said there was reduced capacity on the ferries due to covid but they seemed pretty packed to me.
Either way to the OP i looked into this and i think its nigh on impossible to book tickets in advance but very easy to book the return when your there. Also don't do what me and everybody else did and look at the old timetable in the shabby closed kiosk at the start of the pier and panic, for the fast ferry at least, tickets are sold from the kiosk at the end of the pier.