The person doing their best may still be doing something you never requested.

Effort and usefulness are separate measures, and confusing them makes certain conversations very hard. Someone can work sincerely and at length on the wrong thing, and saying so feels like an attack on their character, because they experienced the effort while you experienced the result. Both accounts are true at once. This went differently from what I needed is not the same sentence as you did not try.

When you have to raise something like this, name the work first and the mismatch second. If you are on the receiving end, hear the second part as information rather than as a verdict on you.