garage door alignment
All the suggestions posted are useful. You need a starting point, Is the door panel level? Check the floor. Is it level? This IMO will start you in a logical diagnosis on what is wrong.
Should the door not be level. Why? Spring tension, rail alignment, binding, or something else not mentioned.
Should the floor not be level.. They make a floor leveler mixture for concrete, can you cut a tapered shim, cut the door panel (if it is wood) , add a aluminum channel under the door.
Hope I gave you some useful ideas ...russ