back door won't lock.- Laguna estate Rt 2.0 1998. Help!?

Central locking system. Back (right side) when locking system is activated, right back door does not lock (at all). However, back left side door, locks, but, does NOTun- lock when c.system is activated. Can anyone who's experienced this prob, with this type of car, help with helpful answers how it can be fixed?? Much appreciated. Mrs B.