Maybe the results are not significant. Not as exciting as positive results, but negative results are still results. I beleive that is fine for a PhD.
Is there obvious trends? Do you see say a decrease in a response but it is not significant? You can still discuss that. Its still a decrease. After all, all statistics do is give a probability that what you are seeing is a real change. They dont in themselves prove (or disprove) anything. They just add weight to your argument (though admitadly quite a bit, and usually required for publication). You could then discuss how you would go about confirming the result is true, or how to imporve the method, or collect more samples etc.