Yep. Been there.
Page 1 is main scheduler page. If you see a location you like with available slots, click it.
Page 2 is the date/time selector. Most of the time it says "There are no available time slots" by the time you get here. If you CAN select a date/time, you move on to..
Page 3 where you actually get to enter your name, phone#, email, age, etc. It asks for your insurance info, but you can skip that if you don't have it handy. There's a 10 minute countdown clock to complete this page. When you're done here, you have a confirmed appointment.
I've had Page 1 on auto-refresh every 13 seconds for the past 4-5 hours. No big blocks of appointments are coming though today, at least not yet. Seems like the slots popping up today are cancellations of earlier scheduled appointments. Slots are gone is seconds. Most of the time, by the time you get through to Page 2 it's already booked. Other times, it tells you to select a time/date, but even then, it's already gone before you can finish. If you can get to Page 3, you are golden.
I scored one appointment for a friend today. Gulfgate HEB in Houston.
These one off appointments are literally gone in a matter of seconds.
Bernard