fruitless
Not giving any results, even after trying hard.
Fruitless means producing no useful results despite effort. When something is fruitless, it's like working hard to plant a garden but watching nothing grow: you put in time and energy, but you get nothing back.
A fruitless search means you looked everywhere but didn't find what you needed. Fruitless negotiations mean both sides talked and talked but never reached an agreement. A fruitless attempt to fix your bicycle means you tried several approaches but the wheel still wobbles.
The word comes from the idea of a tree that grows leaves and branches but never produces any fruit. All that growth looks promising, but in the end, there's nothing to harvest. When we call an effort fruitless, we're saying it turned out the same way: lots of activity, no reward.
Notice that fruitless is different from impossible. Something fruitless might have seemed possible, even likely to succeed. You might spend an afternoon on a fruitless hunt for your missing library book, checking every room in your house. The search wasn't impossible (the book might have been there), but it turned out to be fruitless (you never found it). The word captures that particular disappointment of trying hard but getting nowhere.