As Anebel said, riding up to the contact with your body will be very useful. Try to think of riding your hips to your hands, and your hands to the horse's ears. Forward, forward, forward.
Shoulder in will also be helpful, as it will encourage the horse to accept that contact with correct bend.
In some horses, they have had it so drilled into their heads to drop behind the vertical, shorten their neck and avoid all contact, that it comes to the point that riding them around like a pony club horse, above the bridle, just going forward and gradually taking a stronger contact and building on the roundness.
These horses are very hard to 'fix', which is why you see it preached so often 'Ride the hind legs forward, not the bit backwards'. People raise their hackles if they get told that their see-sawing, false 'head set' is horrible and a very bad idea, but it's a piece of information that will make life much easier for them in the long run.