Paytm Money Java Developer interview questions

Question, which I got in written round was: Convert roman number to numeric digit. I was confident that I wrote the working code with all the corner cases tested. My sheet was little bit dirty coz of my handwriting. After few minutes HR just came out and said your code will not work for 80% of the test cases. I couldn't believe because I was knowing that I have tested all the cases. I wanted to know which testcase will not work and in the answer from HR I got we can't do that and said thank you. After coming to to home, I have just tested the same code which I wrote in the paper on geeks for geeks and it gets passed with all the test cases.