Do Vigin not keep one or two spaces unreserved?
I don't know but many Virgin trains only have 2 or 3 bike spaces altogether so it would be annoying if they didn't allow all of them to be reserved! Why put people off who want to book in advance just to keep spaces free in case somebody turns up on the day with a bike?
I've reserved a bike space before now and then got on the train to find that all the available spaces have been taken. You are supposed to attach a copy of your bike reservation ticket to your bike to prove that you are entitled to the space.
Trains can be very overcrowded and it isn't unusual then to find the bike spaces full of suitcases, baby buggies, whatever.
I got on one Virgin train and found that it had an old-fashioned "guard's van" and my bike was put in that. The staff changed at Birmingham New Street and I then spent an anxious 10 minutes running up and down the train looking for the new member of staff with the key to get my bike back out at Coventry. I had visions of either being stuck on the train with my bike and ending up in London, or getting off the train to speak to someone on the platform at Coventry and the train then leaving with my bike still on board!
Another time the train didn't included a carriage with bike storage so I had to bungee cord my bike into a wheelchair space and sit next to it, having promised that I would be prepared to move it if a wheelchair user needed it.
NB - it is always a good idea to carry a couple of bungee cords when taking your bike by train. You can stop the bike swinging about from the storage hooks. Alternatively (as above) you can secure your bike somewhere else if the bike storage isn't available. A lot of local railway services don't allow you to reserve bike space so it is quite common to have to improvise on them. You might struggle to get a bike on one of them during peak travel time.
I've never yet failed to get my bike on a train, but I did fail to get myself on a train just after Christmas when I didn't have my bike with me. It was so seriously overcrowded that the station staff refused to intervene. They said that there would be a riot if they tried to get 100 people off the train so that 100 of us on the platform with seat reservations could get on.