Like here - OP did not provide MySQL error from it's error log, and without it it's impossible to know. But instead of finding out, he posted a duplicate, still without that log message. Now, it's obvious what to do with duplicate. But what with original question? In situations like this we can't even know if it's on topic or not, so should we downvote after some time? Vote to close as what? Or any other action fits that situation?
If you feel the question is of poor quality/not useful for the site/etc, then downvote away. Similarly if the question in its current form is off-topic for the site, vote to close it if you normally would. If you can't tell whether or not it's off-topic, it probably stands to reason that it must then fall under "Unclear what you're asking" anyway.
Adding comments to request more info isn't mutually exclusive to the above actions, which can always be reversed later if/when the OP brings the post up to standard.
As for the lack of responses, I would say just ignore it and move on. We get that attitude quite a lot, where people will selectively respond to the comments they think will lead them to an easy solution without them having to make any/much effort themselves.
It's frustrating, yes, but unfortunately there are people hell bent on using this place as a personal support forum. IMO we don't need to treat posts that display those characteristics any differently from regular questions.