ford focus central locking?

I recently purchased a 2004 Ford Focus 1.6 Zetec. When I try to lock the car using the key fob the lights flash etc, but I can still pull all doors open. I have found that if I lock the front passenger door from inside first then press the key fob it will lock all doors ok ?
It is a bit of a pain as I have to remember to do this every time I leave the car.
Any ideas on how to remedy this will be truly appreciated. John.