I'm having problem triggering Siri when I have the Apple Watch, my HomePod mini and my iPhone all together. I'll say "Hey Siri" and I'll see all three devices come on, and then all three devices turn off. Doesn't happen all the time, but when it does happen, it happens consistently. I can say 'Hey Siri' 3 or 4 times in a row and all 3 pick and then all three stop, I assume thinking one of the other devices have it.
Another problem I had today was that I have the Apple Watch up to my mouth and I said Hey Siri and the watch did not respond. Then I say Hey Siri again the phone in my pocket says "Uh huh?"
Don't use it. I keep forgetting it's there. If I force myself to use it, I'll develop the muscle memory and I will find it to be more useful.