Yes, I'm seeing the same thing. It works 99.9% of the time if it was used recently but less often if the Sonos has been inactive for a while. I'm not sure if it's because the Sonos itself is going into a sort of sleep mode, or if it is the connection to the network share that slows down. Either way it can probably be fixed.
As a test can you try to insert a Sonos.Player.GetVolume action in front of your TTS action to see if it forces the Sonos to wake up first? If we can verify that it works I may just put that into the TTS action. It would probably be good anyway to add a volume parameter to the TTS action, in which case the plugin would get the current volume, set the TTS volume, play the TTS wav, and then restore the original volume. Of course you can do this now with actions but why not make it easier, especially if reading the volume fixes the TTS problem.