When your vet cannot find a reason for the lameness it may be caused by a subluxation in the houlder and or neck. A common subluxation in the shoulder is when it locks in a cranial position ( forward) as a result of jumping or quick turns.
In the neck a subluxation can alter muscle and nerve function to the front limb.
Chiropractic adjustments along w moist heat to the affected joint as well as some stretches will help.