The dog lacks confidence in its own ability. Though it can jump, i'll bet it has not been schooled to jump and therefor, when under pressure, takes what it thinks is the easy option.
Back to the drawing board, start very small and increase week by week. Use wire, chicken is best, as it stretches easy and increase it bit by bit over the next few weeks. Have it jump all sorts of obstacles BUT don't over match it... Confidence is key!
Always use a word when training (I use 'hup') so the dog knows when its jump time.
Make sure the dog see's it as fun, so don't over match it and put pressu