i had the similar problem with my macbook air. later found out that the laptop is so thin that it literally bends when carrying it with hand when transporting. i just had to bend it the opposite way very gently and carefully and worked every time.
meanwhile when i got my macbook pro 15" due to air's slowness, it appeared to have the similar symptom. but it wasn't the mac it was my wooden desk that is not perfectly flat.
try on glass or other surfaces that you might know that it is flat for sure before stressing yourself out