Thinking the issue may be related to too many devices on the wifi network, I purchased a Sonos Boost. It had no effect.
Has anyone experienced this issue?
Best answer by Airgetlam
But when they do "start", are they in sync with the other rooms?
If that's the case, I'm thinking that there's some sort of wifi interference that's causing them not to start up until the speaker can get the data from your system....but that's a guess.
At the very least, I'd try a power cycle on all of your Sonos speakers. I'd actually recommend unplugging them all, then rebooting your router, and then plugging them back in, one at a time, waiting about a minute between each one, in order to reset their order in your router's IP table.
But at the end of the day, I'm just guessing here. You may want to recreate this event, and then submit a system diagnostic, and either post the number here, or contact Sonos to discuss it. There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.