Central locking fault

[Ged116]

Hello,

I recently purchased a second had Ford Fiesta 1.4 zetec (2007) for my disabled wife as a present. However we have discovered a fault and wonder if anyone can advise how we might solve it. When we press lock, on the key fob, the drivers door does not lock, it will only lock if we press the double lock icon. However this doesn't always work and can take many tries before it locks, sometimes we give in and lock it manually. All the other doors lock, it's just the drivers door that is awkward, I wonder if anyone has any advice they can give us? Kindest regards. Gerald