Central locking is picky

[ApproAaron]

Hi all,
Im fairly new here so not sure if this has been brought up before.

I have just got my berlingo and have noticed that the central locking tends to pick and choose when it wants to lock all the doors.
Quite often it will lock both the driver and passenger door but not lock the rear doors. I have to unlock it again open and close the back door again then lock again for it to work.

Does anyone know why this might be happening and a fix for it ?

thanks,
Aaron.