Make circles around them with your horse, allow them to see it at all different angles. You can do this on the ground or on their back. Don't stop them directly in front of the jump, that may be teaching them it's okay to stop and look at things underneath jumps. Maybe lunge or free jump them over them. while your on add leg, sit up, and look ahead.
Help him/her get used to it, lead your horse and bring it closer and closer every time. Let it sniff or smell it and ride near it more.