The first thing I would do is get her looked over by a vet to see whether or not it's a physical problem, as it often is. Reluctance to flex through the neck could be a result of an issue with the neck/poll, or a problem further down the back. Saddle fit is another thing to look into. If she doesn't flex, then she doesn't bend and it could be that the saddle is pinching her. I'd also get her teeth checked.
If physical problems can be ruled out, then it's just a bad habit. Use props in the arena such as barrels and poles to encourage bending, use varying sizes of circles. Opening up your inside rein (towards the centre of the circle) can encourage the horse to flex in that direction, as well as crossing your outside rein over the neck.
Before you start doing that though, I'd look into physical problems first. Any horse I've seen do that so far is due to either a current problem or an old problem that the muscles were never released from.