Best answer by ratty
Presumably the speakers are grouped. If they aren't grouped, and you start the same source (e.g. a radio station) on them separately, they won't be in sync.
Assuming they're correctly grouped, try building the group starting with the other 'room'. If the problem persists, and/or is dependent on the 'coordinator' (i.e. the first room in the group), then submit a diagnostic after a loss of sync and post the confirmation number. It could be a wireless problem but there is a rare hardware fault which can cause this issue.