I think always the rotary cutter should be held straight up & down, not at an angle when cutting. You get a cleaner cut that way. And when cutting several layers at once, if you cut at an angle, probably the pieces won't be precisely the same size.
Perhaps some blades might not be as sharp as others. The package says these are made in China, if that makes any difference. I just got my HF blades & the first one is great! Really sharp & cuts cleanly. Just what I want.
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