I have used the online system and I think the problem is that so many people are online at once. This causes the system to "crash".
I had to book for two elderly relatives. In one case, I gave up and used the phone line. In the other, I managed to book online.
When I spoke to the lady on the phone to book, she told me there were teething troubles but the system is improving every day and they are getting more people in the call centres to manage to phone lines.
No problems in booking. However, I quickly and efficiently booked on line for appointments following receipt of the NHS letter. However, as this was at venue 25 miles away and I received an invite from GP surgery to get this locally - I, obviously, went to that one. Then could not find anyway of cancelling the original appointment on line (as I had booked).