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I saw that a lot back in my snowmobiling days. Someone couldn't their sled started so they took the plugs out, looked 'em over good, and put 'em back in because they looked fine. Still no start. Put in a new set of plugs and it would start right up. Never could figure out what the actual reason was, but whenever I encounter a mysterious no start problem, a new set of plugs are high on the list.
And Dale's right; some plug brands were more prone to this than others.