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I suggest that you check the last 4 inches of track, and the drive (whether it is a chain drive or a cog drive) for any sign of resistance or blockage, bending, etc., that might cause binding or resistance of the closing of the door at that point, about 4 inches from closed and closer to closed also.
After that, you need to look on the Chamberlain drive, and see if you can find a dial for 'Force' or 'Strength' and set it one number higher from its current setting, then try closing the door again. The door opener motor might be stopping because it senses just a little more resistance at that point in the door/track/drive, and stops because of that. A higher force setting would overcome that added resistance (and compensate for any weakening of the drive motor with age, which can and does happen with the passing of many years of use.)