Hi Guy,
I was hoping someone might be able to help with your query.
Our first Sprinter was terrible for this problem, and the dealership replaced the electronic part of the central locking several times.
Unfortunately I have no idea of the work they actually carried out. With our new Sprinter this 'two or three attempts to lock' symptom re-appeared but this time it was due to a poorly adjusted sliding door.
I wish I could be more helpful, sorry.
On a seperate issue, if you have the twin opening rear doors, make sure you keep the hinges well lubricated. The metal of some of these hinges react with the steel of the bodywork and can 'fur up' terribly.
Regards,
John