4.4a Objection to a mark set by the leader can only be made by the leader’s opponent after the first bowl has come to rest. If the opponent objects before the leader’s first bowl has come to rest, the leader may have the jack, and bowl if played, returned to have another attempt at setting a mark. Only the opponent can object to the mark. An opponent objecting to a mark must make a clear indication that an objection has been made.
4.4b If, after an objection to a mark has been made, it is proved by measurement to be a mark, the jack and bowl shall remain.
4.5 If the leader fails to set a mark, the leader’s opponent is then entitled to an attempt at setting a mark. In pairs this will be the player waiting to follow the leader.
4.6 If the leader fails to set a mark, objection to an attempt by the opponent must be made by the leader before delivering the first bowl.
4.7 If the opponent then fails to set a mark, the leader shall have another attempt and so on, alternately, until a mark has been set by one of them. The original leader to play the first bowl.
Laws of the Game from the British Crown Green Bowling Association