You find out your idea has been tried, five different ways, all of them failed. Two of your follow up ideas were already done, and had some impact but not as much as expected. Finally you have an idea nobody has done, you try it and it flops—maybe somebody had tried it before but their results were so bad they couldn’t publish.

Then in the end you have done a tiny increment over the state of the art, and you want to persuade the PhD committee this is enough and they should let you graduate.