Why would a dr not be able to perform a lachmans test?

Pain, Swelling, etc.: The most common reasons would be that the patient is in too much pain at the time of the exam, the muscles have tightened/spasmed to protect the joint, too much swelling to allow any movement, etc. There may be other contraindications, like a possible fracture/broken bone. Sometimes, it is just simple mechanics, where the patient's size prevents us from doing certain tests.