I guess this might have been already covered somewhere on the Meta, but I have no idea on how to search for it - if so, please point me to that question.

My problem is that I've noticed that since several weeks I get very often upset about answers, that are really great, but are not marked as accepted. In case the user has 1 rep, and it seems that he'd have never touched the site again after he had asked the question, I can excuse them. But if the user who asked has ~100 or even ~600 rep points, and he actually comments that this answer worked/helped him perfectly, but does not mark the answer as accepted, I find that a bit worrying.

• The first reason is that AU would be somehow easier to maintain if the questions that are answered and solved were actually marked as such. This also negatively affects the statistics :)
• Another thing is that very often these answers are written by users of 50-300 rep points, and in their case the +15 points they would get if the answer was accepted really matters, and is also a factor that encourages them to contribute to AU further. (I feel like writing obvious things, but I have a strange need to explain my idea clearly ;-) )
• And of course, in case of users that have 500+ rep points, such cases lower their accept rate, and they definitely do not want so, but probably do not know about that.

Some quick examples: one, another, one more.

I guess that the cause may be that they are either not aware of the possibility to accept answers, or are not aware of why this is important (It's not that they are ignoring it completely, because very often they sort of accept it by writing a comment of praise, relief, or express thankfulness). Usually I write a comment, encouraging to use the grey/green button left to their favorite answer, with a short as possible explanation of why it is good to do so - it helps, but only occasionally.

Yet I am not sure if my approach is right, and would like to consult how to deal with such cases. Should I even care? And if yes, is there any effective way of encouraging such users to accept the answer they like?