Hello,
I've been having this problem basically forever, but I intermittently when trying to activate X10 Lights via HomeKit, I get by Siri "Some of your devices are not responding, please try again later", or sometimes "Your devices are taking a long time to respond". Try it again or try it twice and it works. Looking at HomeKit, you can sometimes see all of the devices say "No Response" but refresh the screen (swipe left or right) and then they are responding properly and work fine. It seems that HomeKit Bridge goes idle and HomeKit has problems connecting to it when it does this. Once it 'wakes up', it works perfectly and has no issues. I never have problems though when I use the location stuff in HomeKit to turn the outside lights on when I get home for example. I also use HomeBridge and that doesn't seem to ever go into No Responding status for my devices which are outside of Indigo.
Are there any settings which can be tweaked for this? I'm surprised other people don't seem to be having this issue, it would be nice to fix.