So, you think about new things that you can do with vacuum cleaners. So, first, you're like: I can use a vacuum cleaner to clean bookshelves! That'd be super-useful! But then you do a Google Scholar search and it turns out that someone else did that like ten years ago.
OK, your next idea: I can use a vacuum cleaner to clean cats! That'd also be super-useful. But, alas, a bit more searching in the literature reveals that someone tried that, too, but they didn't get good results. You're a confident young grad student, so you decide that, armed with some additional techniques that you happen to know, you might fix the problems that the other researcher had and get vacuuming cats to work. You spend several months on it, but, alas, it doesn't get you any further.
Yasha Berchenko-Kogan answers to What do grad students in math do all day?