It’s usually a judgement call. Sometimes a thread has gotten so long that most people won’t bother to tackle it. You can usually get a decent number of folks to check out what’s new that bumped it back up, but if it’s a continuation of the discussion of the thread, it’s possible you won’t get many new responses, as people aren’t as likely to try and read through enough of the posts just so that they might join a long conversation.
However there are others, like this one here, which was just bumped today after nearly 3 years…the new post isn’t responding to anyone in the thread, it’s just a response to the OP. And you’ll see it’s gotten other responses today too. Another big part of it is that it’s just not that long. You’ll see that I actually didn’t notice right away that it was actually as old as it was.
So if the thread where your issue is already discussed doesn’t really answer your question, and you feel it is so long that many people won’t really look at it, go ahead and make a new thread, but link to the previous one and explain what it is about that thread you didn’t understand, or why it didn’t answer your question.
This will help people not only better understand how to help you, but it will also make them more willing to do so, as they see you’ve made the effort to seek an answer for yourself first.