When you first add a new speaker to the group, (or group all speakers using the Play Everywhere button) it is normal for all secondary speakers to take am moment to buffer before beginning to play. If your are instead experiencing lost of sound in grouped speakers mid-playback, this would be most commonly caused by insufficient signal strength. Start by checking on what RSSI value those speakers as mentioned in this article. We suggest a "good" or "great" RSSI rating for all speakers involved. While weaker signals may be enough for individual playback, grouped play requires substantially more bandwidth. Make sure also that the speaker with the strongest connection to the router is the first one playing so that it will act as the master device for the group. Hope this helps!