Rear Child Locks

For the first time since new (15 months) I had a couple of passengers in the back yesterday - I was surprised to fine the rear offside door child lock was 'on' and despite my efforts to reset it I can't, everytime the inside handle is used the child lock comes on - anyone come across this problem before?

It might be that I'm not turning the 'off on' mechanical actuator far enough but I don't want to force it......?