Central locking intermittent but locks after unlocking rear doors

[ramemory_]

Hello All,
I have a 2012 Citroen relay and am having issues with central locking. The van thinks that the doors are locked (I'm not getting the quick flashes to indicate a door open like the usual), but it is not locking the front doors all the time. It is very intermittent, sometimes it locks both fronts, sometimes only one of them. sometimes none of them.
The door locks also make a noise every time I press the lock button, but seem almost as if they haven't got enough grunt to fully lock. and I can just open the door from outside.
The only way I can reliably get it to lock is if I unlock the rear doors first. then lock the whole van together.
I thought maybe it was the wiring in the doors but the fact it reliably locks after unlocking the rear doors doesn't make sense to me... Although this is on my list to check...
Any help would be appreciated.
Thanks,
Richard.