Combining these to show that, while not impossible, writing a system to connect multiple, disparate systems is not easy.
I’d guess that vaccines.gov gets a periodic feed (weekly?daily (my bet)?hourly? (real time is highly unlikely)) from the major providers - which probably worked fine when there was a basic balance between supply and demand. Opening the doors to everyone for shot #3 and add in all the kids presumably shot that to hell.
I’m surprised that this is still going on. After reading the OP I checked the date assuming it was a 6 month old zombie, since it perfectly describes my experience trying to get the vaccine back in March and April. Now when I call my pharmacy I get an ad saying they all have vaccines and walk ins are welcome.
Of course I live in a vaccine utopia (99/9% at least partially vaccinated baby! Gloat Gloat). So my experience may not be typical.
In Northern NJ, I just walked into a giant old Sears (now a vaccine center), said I’m looking for a ModeRNA booster and walked right in. No appointments, no lines. Took about 25 minutes total, including the 15 minute post-shot monitoring period.
It’s not that high here, but >90% of the population in my county is completely vaccinated. The issue is that the booster just opened up for the entire adult population and thus there’s high demand.