I know that this is a generalization, but why are people with knitted kipot usually friendly and helpful, whereas people with black hats will not even answer someone who says "shabbat shalom"? I see this problem mostly with young people. Don’t the black hat yeshivas, or black hat parents, teach manners anymore? [Note, I’m not talking about greeting people who are dressed immodestly, but would be interested in knowing your thoughts on how to handle that as well]. Thank you.

I cannot confirm or dispute your observations. It is possible that in certain areas children (I don't know if you are talking about children or young adults) are more cautious about speaking to strangers than in others. You are certainly correct that in the absence of safety considerations, everyone should greet his neighbor warmly- Kol Tuv!