I totally agree that any of my answers can be be downvoted. The problem is that there is no way to know why. Is there any manner to ask somebody (may be the moderator) to clarify and tell me what I set wrong?
It is much rarer for answers to be downvoted than for questions -- the predominant culture here is to comment when an answer has an error, rather than to silently downvote. Here are a few reasons I might downvote your answer without comment:
It shows a lack of presentation effort: there are spelling/capitalization errors, math is not Latexed, etc. I would guess this is the reason your recent answer got the downvote.
The answer is wrong. Usually I will comment and point out the error, but if the error is egregious (suggests a hopelessly wrong approach, for instance), or you didn't respond to another user's comment in a timely manner, I might downvote instead.
The question asks for a hint, and you spoil the full solution.
If you want to ask, in a comment to your question, why you received the downvote, it is OK to do so. But you won't always get an answer.