I am a master student, working for X lab of Y university as a research assistant.
3 months ago, a post-doc(my direct supervisor) recruited me into a project when he wanted to submit his paper within a month, telling me "You can coauthor if you join." Since then I worked overtime to finish exactly what he told me to do, implementing all his new ideas (but none of them worked). Later he gave up previous submission, had the project updated a bit and now preparing submission to another conference in a month. Last week I asked "Am I on your author list?", only to get "Let me think about that." Project is almost finished currently, in terms of both experiments and writing.
Now I have no idea what to do, his reply sounds like a NO to me. To fight for that, I don't have any strong argument other than his vocal promise. My contribution is not direct, but a part of code he used was written by me (for another project), and I successfully implemented his new ideas.
Given such situation, should I ask him again for this authorship? Or should I explain everything to PI? If then, how to?