Toilets can appear to have flushed fully, but still have...material...stuck in the U-bend that hasn't completely evacuated the toilet. A subsequent flush won't work, even though the water in the bowl is clean.

Ask me how I know.

That said, this could almost certainly be better-solved in other ways. Maybe by preventing the tank from refilling if there's still something in the u-bend (then you'd know it needed attention because there'd be no water in it)?