I am a PhD student in Mathematics. My advisor proposed me a problem 6 months ago, and I did a lot of progress on it. I have reached a point where I have quite substantial results (admittedly, adapting techniques from other areas, so not a great deal of originality). They can surely form a paper, though not a great one. The original question remains unanswered in its full generality, but I felt that a complete answer would be impossible from the very beginning, and now I have very concrete reasons showing that a complete answer is indeed very hard (in the sense that a general answer/formula might simply not exist). My advisor wants me to keep working on this, but I honestly don’t know what to try anymore, and he did not suggest anything. I guess this is a common occurrence during a PhD, what should I do?