If your chains is fine, and the bar is straight, then the bar probably needs grinding. It needs two square and not rounded shoulders exactly the same height. Yes, you can got it with a file, but it's a real pain - just take it to a saw shop - it only takes about 5 minutes.
If you have a wood working disk sander, the type that has vertical disk about 10-12 inches, and a table at 90 degrees, you can do a pretty good job. Lay your bar down flat and grind in nice long even strokes. Clean up any edge burrs with flat file.