I'm a historian, so the work is different, but the overall theme of the responses here resonates with me: read. That's how you find the issues and pieces of evidence that have been neglected, the controversies in your field, and learn the different approaches that previous scholars have taken to problems. The more you read, the more you'll see.
I'd also echo other responses here in saying that I rarely find a new issue reading meta-analyses. Going to the source (whatever that is in your field) is key.
Again, my work is different, so the utility of my response may be limited. I hope you find a problem that intrigues you and makes your work fun!
I'm in a totally different field and this is 100% my experience. Some of the old papers have really great ideas that no one ever worked on, or talk about problems, which we have more data or better solutions/attacks on now. Finding these are like finding gold nuggets - they can set you up for years.
I'd also echo other responses here in saying that I rarely find a new issue reading meta-analyses. Going to the source (whatever that is in your field) is key.
Again, my work is different, so the utility of my response may be limited. I hope you find a problem that intrigues you and makes your work fun!