I'm quite interested in seeing video footage of how you approach the fence.
Faults, Refusals, Veers, etc,etc occur due to 99.9% of rider error, so lets stop and ask what is it that the rider could of done to of created this constant outcome in her horse.
1) Horses jump blindly. Facts are that they cannot see the fence 3 strides out. The fence has completely dissapeared in their line of sight, so they now turn and rely 100% on their rider to get them to the base solidly, to aid them over the fence.
If the rider does one thing incorrectly from point A to point B - errors occur.
I have seen countless times, horses jumping regardless of what errors the rider commits while in the saddle. Those horses are saints, and get their job done regardless and those horses deserve all the praise they can get.
Then, there are those horses, such as Nelson and many more - that wont. If their rider faults them in any way *shoulders, legs, seat, upper body, hands, eyes, head* on approach to the fence - they will not jump it. What Horses Really See When Jumping
So, lets stop and look at the OP's first, before pointing the finger at the horse.
Video's would be helpful to see what exactly is going on.