Apple

Yes I have had similar experiences, the one time I did lose consciousness and fell down and got a concussion the watch didn’t do anything. Luckily my wife found me soon after and called 911

Apple Watch cannot detect all falls

I fell hard once, caught my toe on a step and straight down, watch didn’t register it. Thumped my husband’s arm in jest one night and it thought that was a fall though!

i have fallen and it does nothing - and yes i was still long enough that it should have

I fell hard about a month ago and fall detection activated! I think this was the third time I fell or landed hard against something and it came up

Last night I had it randomly go off🤦‍♀️

I fell once and saw the options come up for “I’m ok” or “call emergency” but they disappeared as I was getting up, does movement tell the watch that you’re ok so it dismisses the fall notification?

I believe it only works if you're motionless for 30 seconds or so after the fall.
Since you fell down an entire flight of stairs and appearantly lived, congratulations. But again since you fell down an entire flight of stairs I suspect the trouble is with you and not the technology. Just kidding.
But perhaps we've become a little too reliant on technology.
There are news articles about Google Pixel phones not calling 911 when the owner has dialed 911. Talk about a sinking feeling from something most take for a given it'll work.

https://preview.redd.it/u9v2e3atk02c1.png?width=960&format=png&auto=webp&s=893adff1640b63ece8deebdb4e0af33ff6a0db58

You need to make sure you only fall these ways next time.

Ditto here. Thought it was just me. Had lots of false fall alarms over the years, but never an alarm with a real fall. My one big fall with no injury that did require a 911 call was a very slow gentle fall. I thought because it was a gentle fall the alarm did not trip. I did however really need and used the ability to call emergency from my watch as I could not get to my phone(s).