Sometimes originality comes from the approach you are taking to making links between different things - for my PhD, the novel aspect is in the combination of factors that we are considering in the context of two problems. They have individually been looked at extensively, but little has combined them.
Maybe you need to have a closer look at a specific problem, and at some of the proposed answers/solutions to that problem? If it has all been said already and you need to say something new, build those established answers into your thesis and argue against them - what are the alternatives? Is there another way to approach this problem? What is a different angle? Have the methods others have used been quite similar, is there any way you could approach it using a different methodology?