When you flag someone for potential voting fraud, a moderator will always look into it. How long they spend looking at and the ultimate outcome it kind of depends on the case.
Initial Look - Does it look like there's even a problem?
We get a lot of people who get two or three downvotes and then go off flagging their post complaining about "serial downvoting" - this is pretty much a waste of everyone's time. If we look at the reputation history and don't see any real signs of mass voting, we will probably just decline the flag and tell them to get over it. It's not worth it to get the SE team involved to reverse those votes, which may very well be valid.
As well, flags which are vague and do not indicate when the events took place may end up declined if the moderator doesn't find it right away.
Investigation - How many users and votes are involved?
Once we've determined something is probably up, we have additional tools at our disposal to investigate how many votes outside of the obvious ones might be fraudulent, as well as how many users are involved with the fraud. We can look up users, IP addresses, and emails to compare users and determine if they're sock-puppets, friends upvoting each other, or maybe just some random person not understanding the system. What we find out will influence the final action we take.
Final Action - How do we solve this problem?
Moderator Message - If we feel the user who received the votes is directly involved with fraudulent activity, they will almost always receive a message. Depending on the severity of the case, or if they've done it before, it may be accompanied with a timed suspension.
User Deletion - In the case of sock-puppets, this is by far the easiest solution as it does not require us to get the SE team involved. It lets us quickly clean up any fake accounts and all the votes that came from them.
Manual Invalidation - This involves contacting the SE team, as a moderator cannot actually invalidate votes. Usually we will only bother the team for more major cases where there are quite a few votes, or multiple users involved (such as a voting ring).
As in any other area of the site, moderators sometimes make mistakes. Sometimes we miss things, and a fresh pair of eyes on the situation can bring on a different outcome.
In your particular case, action was taken and the flag was helpful. More action could have been taken, as the problem was actually a little bit larger than what was seen at face value. That's been resolved now.
