Covid vaccine booking question

[LivinLaVidaLoki]

Sorry I'm probably being a bit thick, lack of sleep.
Currently if you are in one of the priority groups you can book both your first and second dose online. If you've already had your first, why can you not book your second?

You will get your second appointment when you’ve had the first, they put it on a card the date and time. You have to have second at the same place as the first.

When I had my first they just put the details of the first, nothing about the second.

Some sites are not yet booking second doses as uncertainty of vaccine supply. You should be contacted to book in nearer the time.

Booked mine a couple of days ago on the NHS site (over 65 group) and it automatically took me through to book my second appointment in 11 weeks. DH had his 2 weeks ago (over 70 group) and he booked through a text sent from our surgery. No second appointment booked yet.

Booked via NHS website at mass vaccination centre, choice for second appointment offered at the same time.