I had the same problem with my M2 in soybeans.I turned the knives around before the start of last season and didn't have any more problems.I decided that as soon as the plugging starts again I would put on new knives.
Had no trouble once I turned the knives around, that is, till a bearing went out on one side of the chopper this afternoon. Got that fixed though too. Was cutting some 1506 milo, sometimes with the header all the way up in spots. You guys that grow milo probably know what I'm talking about. Yields pretty good and stands remarkably good for its hieght.
Dull chopper hammers plug walkers. To sharpen and keep rotor balanced remove one hammer from each row on rotor- clamp together grind square without getting to hot. Replace on rotor and repeat until all done. Removing chopper concave and sharpening teeth helps also.