This could be as simple as the gear linkages being out of adjustment - so the gear isn't being allowed to fully engage in the first place. This usually occurs if the gearbox has been disturbed to fit a new clutch, for example - or maybe its just got a high mileage on it. Best visit a Ford dealer or good independent garage for this one.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.