RE: One way bearing or just the pull start itself???
Yeah more than likely it's the one way bearing. Try pulling the string slowly and and see if the engine turns over multiple times and that it's gripping the shaft during each turn. Versus pulling it fast and it only turning over the engine once, like you said. If this is the case, then definitely it's the one way bearing slipping. As this is generally the problem, especially if there's fuel/oil soaked in the one way bearing.