That has been the story of Mercedes, especially with Lewis Hamilton. "Best car set up for ages", goes well in practice and then doesn't in qualifying, or goes well in practice and qualifying and falls apart in the race.
The design of cars at the moment seem hard to set-up and Mercedes have not cracked it. What they are seeing in computer simulations, and sometimes in practice sessions, they are not reproducing in race conditions. They started with a radical design, hoping it would be much better than the rest. That did not work, but they took the attitude it "should work" and persevered when perhaps they should have tried something else. They have put a lot of effort into something that did not work and now they are trying to catch up.
I suspect it is a bit like clay pigeon shooting. Shooting well and hitting the clays dead centre, all is well. Slightly off, and maybe you hit one slightly low, high, behind or front, doesn't matter, still counts and you get a indication and a warning that your shot was slightly off and you know how to correct it. But, if you are missing and you have no idea whether you are going high, low, front, or behind the clay and even the best shot will have a problem. This is I think Mercedes, they don't really have handle on what their problem is.