Did you get your first shot at one of the chain pharmacies? I’ve actually been fairly happy with how efficient Rexall has been so far. I put my name and preferred location into their system the day they announced the drop in eligibility to 40+, and 72 hours later I was texted with a list of appointments from which to choose (this was in the peak of “boomers are freaked at the blood clots, get the young 'uns in so they don’t go to waste”). When AZ was temporarily put on hold, they emailed me with a “We’re on it, we’ll fill you in as soon as we know what he province decides.” message, which I appreciated.
The August 19th date you got was possibly not a firm appointment date; my official record sticker that I’m keeping in my wallet says “Follow-up dose eligible after: Aug. 7, 2021” so it wasn’t carved in stone at the time.