I have seem some occasions where a newly-baptized member doesn't appear in the ward records for a few weeks. Until the member's record appears, you don't have the MRN. If the member doesn't appear in LCR yet, then the ward mission leader should follow up with the mission. (It sounds like this is a convert baptism and not and 8-year-old baptism...wards, not the mission, are responsible for recording 8-year-olds.)