My Focus is a Mk3 TiX bought new last year, and I initially had all sorts of problems with the keyless entry.
That is until on a visit to my dealer, it was pointed out that I shouldn't try and lock it by putting my hand round the handle and pressing the little black window, I was trying to lock and unlock it at the same time! Doh!
The other thing I didn't realise at first is that only the door near the sensor will unlock, i.e. if I stand next to the drivers door the passenger cannot unlock it on the other side.
My excuse is my old age...