What you call an "obvious" case isn't that obvious. While it might be the same user, three downvoted questions in succession isn't much.
Think about the following scenario: You see a bad answer. You downvote it and check the user's profile just to find another bunch of bad answers. You then downvote these as well. Maybe four or more. Was that a "driveby downvote"? Did you do something wrong by downvoting so much? I don't think so. Still, the system will probably revert these votes.
Scrolling down your reputation history, you can actually see that serial downvoting cases are reversed by the system. What happens is that the offending votes are removed on the offender's side and your reputation is restored as it was before. Apart from that, nothing happens – at first.
Since moderators can't look into who cast a vote, they can't help you with that situation either. It just seems that, as Ivo Flipse said, the offending user doesn't do it often enough.
It really happens often and still goes unnoticed, send a mail to
email@example.com and explain your situation.
If there's a repeated offense by one user, their account can be suspended for "voting irregularities" though. This requires human intervention though – so it's nothing the system can do. You'll need someone to check this for you.
See also (albeit for another case of sock puppet voting):
"voting irregularities" is code for "caused voting to happen in a way that would not have happened naturally had he not created fake users or solicited friends to vote on things".