I struggled with this for a while yesterday but finally realised how it's supposed to work. The mistake I made was to expect the car would lock if I placed my thumb on the dimple. It doesn't. I found that placing the back of my index finger against the dimple worked every time. Also doing that immediately after locking with the remote disables the automatic unlock feature - an extra security feature to guard against some scrote using a key repeater to steal your pride and joy.