Encouraging people to explain down-votes
Sometimes there are bad questions that receive downvotes for different reasons.
The "Stackoverflow netiquette" says that, when downvoting, it's a good idea to explain the reasons in a comment, so the user can edit and improve the question.
The question is:
What is the user to do if she is receiving downvotes without any explanation? The reason of downvote can be obvious for the downvoter or the experienced users, but maybe it is not so obvious for the user who posed the question in the first place.
How can a user ask such a thing? Is there any protocol?