An OP has commented that my answer was correct and resolved his problem, but he didn't vote for it, or accept it. Is there anything to be done, or just let it go?
Well, you should not pressure the OP to vote and/or accept your answer; despite the fact that most of us would obviously agree that voting and accepting is beneficial to the idea of Stackexchange - upvotes or acceptance signals an answer is helpful and thus worthwhile reading - and to the users participating (yeah, virtual internet points as an incentive to provide quality content).
What should I do when someone answers my question? Decide if the answer is helpful, and then...
Vote on it (if you have earned the appropriate voting privilege). Vote up answers that are helpful and well-researched, and vote down answers that are not. Other users will also vote on answers to your question.
Accept it. As the asker, you have a special privilege: you may accept the answer that you believe is the best solution to your problem.
Note that it says should not must. In this particular case it is however noteworthy that the OP cannot not yet upvote your answer due to the lacking reputation (in other words the answer itself lacks upvotes in the first place). He could still accept your answer though and I am usually fine with a comment indicating just that.
you should not pressure the OP to vote and/or accept your answer
I'm going to disagree with this, if we take "pressure" to include a polite response. I think there is at least one scenario where it is okay: If this is the OP's first question, or if looking through his/her other questions from the profile link, you notice s/he has never accepted an answer. This is a clue the OP may not understand this part of the system.
Of course, if that person had taken the new user tour and/or read other Q&A's -- both of which I would hope someone would do before asking, but a surprising number of people clearly do not1 -- they would likely have picked up on this. The fact that so many of them do not is testimony to the single-mindedness of people who want help please with their problem now.
That's understandable, and if that person then thanks you in a comment but doesn't tick the check, it's probably because they don't get it. In this case, I think there's nothing wrong with a polite:
If you this answer helped you solve your problem, please accept it by ticking the big checkmark at the top on the left.
I do this occasionally (on my own and other people's answers), and I notice other people having done it, and most of the time the OP does exactly that. Although it may seem aggressive, it is helpful if the OP really didn't understand (and might otherwise continue on asking questions and never accepting an answer). If they already know, I think most reasonable people would take it for what it is: A tip to someone they think needs it. No harm no foul!
You can tell if someone has taken the tour if they have the "Informed" badge. Only ~10% of our members actually have that, despite the fact that you are invited to do so when you sign up (that's not adjusted for people who came from other SE sites where they may have taken the tour). If they don't, and they don't have other SE memberships, that's a clue they did not take the tour.
The majority of people who post questions as answers (go figure), and/or get their first question closed by me for whatever reason, fall into this category (which is why I politely ask them to "Please take the tour...").