another posiblity is that the spings are worn out, causing the "sag" on one side of the door. I would watch the door open and shut a few times to see if there are any binding points, especially towards the bottom, also check the bottom strip as previously stated. A worn weather strip could prove misleading when looking at a reveal.
If you suspect the springs, look at them because they are color coded on the ends, you will see colors like white, orange, green, etc.. that is crucial in a proper replacement for weight displacement to replace them with the proper color, NOT eyeballing size because different companies wind springs differently to handle specified weights. Also, replace ALL springs, not just the two on the sagging side.
If you happen to have the torsion spring across the top, then you may need to troubleshoot the assembly by reading your manual if you have it, or looking online based on the brand of door you have.
If all else fails, call a "certified" repair technician to troubleshoot and repair the door.