Looks good to me. You are right, it's hard to say what's going wrong when nobody gives feedback. Often if you do well on 4 problems and then tank the 5th, that can be enough to miss out. I think if you can regularly solve this difficulty of problem you should be able to get a job though.

Would you mind emailing me whatever resume-ish stuff you have at my hn username at gmail? Maybe I can give more specific help.