On our 377's in the wheelchair area there is a wheelchair sign, with the words 'priority by law'. Has helped, pointing out that sign on one or two occasions.
I remember once, I had a booked wheelchair (they mark them on our digramme). It's sometimes 50/50 as to if they turn up. Some might just get an earlier train, or something else. Anyway, i pionted out to the folks sitting in the space that in a couple of stations time they would have to move. There were other seats available. They staid put. Two stations down, there was the pass and their chair. I go to get the ramp out saying you will have to move. some huffing, but no movement. Get the ramp down, chair on. Slight movement. I said ' I did warn you, and you WILL need to move'. They did, in the end, but not before loads of huffing and puffing. But at the same time, most punters will move, no problem.
I've never left one behind before, and if i spot one, I never intend to. Even if I have to delay the train. There has never been any comeback to anyone who explains this correctly on a 'please explain'. I've even got one on, on a Brighton pride day. Trains packed, poor soul had, had 3 trains stop and go without him. A stern 'we are not going until you clear some space' to the other punters, soon gets a space. I've also known some start placing bikes off the train to create space.
Now, on the other hand mobiltiy scooters. We do our best, but some are just too large. I've had some on (put on before I got on) that took up almost the whole vestibule. I've had them crash into doors, take 2 mins to board. some stations are worse than others. Hampden Park to Eastbourne. You pull up and 3 are waiting. We used to have some turn up with trailers.