Minor Autoclave in Pathology sometimes doesn't clean Petri dishes

[Podrick Equus]

I've noticed when I insert a dirty petri dish (dirty as in unusable for growing pathogens) into the Autoclave and activate it, that the petri dish is still dirty after it's done "cleaning" it.

However, after one or two more attempts to clean that petri dish, it will be successful in cleaning it, which I find very odd.