The only way that would happen is a bug. YC always responds to applicants. Rejections usually aren't personalized because that can't scale - but not hearing back at all should never happen.
If you want me to look into what might have happened there, I'd be willing to try if you email hn@ycombinator.com.
No investor is going to give you specific feedback.
Your best bet is to go to onto Reddit, Discord etc, find the startup communities and ask for advice there. There are a lot of people who want to support other founders. Or even better go on Linkedin, find some prospective customers and ask them for advice. I've had about 10% of people reply back some with page long answers.
If you are willing to spend money there are plenty of services like Kintell, Intro.co which will allow you to book an hour with investors or successful founders. But there are plenty of free options that I would start with first.
Reconsider writing ideas off so quickly! Just completing the application is a great way to test your thinking on each. The process could be enough for you to transform one of them into something that excites you.
The only way that would happen is a bug. YC always responds to applicants. Rejections usually aren't personalized because that can't scale - but not hearing back at all should never happen.
If you want me to look into what might have happened there, I'd be willing to try if you email hn@ycombinator.com.