why have an appointment

phoned and booked my babys 6 week injections. was giving a time and was told that they were going to get stock in. not once on my phone calls was I told that it was a first come first serve bases. I called to today to ask if they could change my time by an hour I was then told that the 10 vaccinations that they got were getting finshed I asked if they could keep me one as I had booked and had an appointment I was told they couldn't cause people will get upset! I BOOKED AND HAD I TIME TO GO THERE WHY COULD THEY NOT HAVE KEPT IT FOR ME