Coupler check list

The first thing to check is the coupler height. If both couplers are too low they can be raised by shimming up the trucks, which raises the car body to which the couplers are attached.  If only one is too low then only shim the truck at that end of the car. If one or both are too high put shims between the coupler gear box and the frame of the car, or there may be too many shims on the trucks.
Parts of the coupler.
Draft gear box and lid, or the case that holds the coupler assembly. Some are held together with a metal snap lid, some are glued together and some have a mounting screw which holds it together and holds it in place. Make sure this screw is not too tight.
Bronze centering spring, make sure it is not bent out of shape.
Shaft, knuckle and knuckle spring. Make sure it has no burrs and moves freely. A hint, when replacing the spring first put a loop of thread thought it so when it goes flying, and it will, you can find it.
Trip pin should be checked and adjusted for the proper height.
When assembled, the coupler should move from side to side easily and center itself.  The lubricate the knuckle and inside the assembly with a puff of powered graphite.