First, make sure the poles are not raised too high. The higher poles should be no higher than the highest height (and no higher than hock height) that your dog is comfortable with in the intermediate level. For example, if your dog is able to trot comfortably when the poles are 1" off the ground, the highest poles should not be higher than 1".
Next, check that the poles are set at the correct distance for your dog. If you had been working on increasing the distance between poles for your dog, you may have to reduce the distance (e.g., start at withers height or twice the elbow height) between the poles.
If your dog is still tripping over the higher poles, decrease the height difference between the "low" and "high" poles. For example, instead of having poles set at 1" and 1.5" off the gound, try setting them at 1" and 1.25" off the ground. If your dog is still tripping over the higher poles, lower the poles to 3/4" and 1".

Troubleshooting