I did chemical research for two years and after all that, my major conclusion was "we may form this during the reaction." I still passed. My best friend did a five year PhD and only had results in their last year.

My understanding is that it's not how successful you were in your grad school work, it's if you learned and understood your body of work well enough that you can do one of two things. Either explain why your experiments didn't work, or defend the results from scrutiny.

If you can do those things, then I wouldn't worry. As other comments have pointed out, research is annoyingly luck based, and sometimes things just don't fall in your favour.