The German manufacturer had come into this year hoping that a design revamp would help it close the gap on pacesetter Red Bull.
However, drivers George Russell and Lewis Hamilton have endured a challenging first two races to the campaign with a clear weakness to the car now exposed.
Team principal Toto Wolff says that the outfit does not fully comprehend what is going wrong in high-speed corners, as its real-world performance is not matching what its data suggests.
"There is something which we don't understand," he said. "We are quick everywhere else pretty much.
"We know that we have a smaller rear wing, we're compensating what we're losing through the corners. But it's just at high speed where we're losing all the lap time."
Wolff explained that the issue goes far beyond just being a set-up problem, as he thinks there is something intrinsic in its car design that is not working.
"I think that's a biggie," he said. "There's only so much you can tune here.
"Our simulations point us in a direction and this is the kind of set-up range that we then choose, where you put the right rear wing on.
"I think you'll gain a few tenths or not if you get the set-up right or wrong, but there's not a massive corridor of performance. It's more a fundamental thing, that we believe that the speed should be there. We measure the downforce but we don't find it in lap time."
The difficulties with the W15 have opened up the prospect of Mercedes facing a third consecutive year of chasing problems, after its woes with the new-ground effect cars over the past two seasons.
But Wolff is sure the squad is in a different spot right now, and he has no doubts it will quickly get to the bottom of what has gone wrong.
"It's been two years that there is something we need to spot, and that's the thing to unlock," he said. "We have just got to work.
"It's not through lack of trying. We've pushed so hard and we're going to give it a massive, massive go now in the next week, with more data to understand.
"We are going to come back to Melbourne strong. We are on a mission on this one. And I am 100% sure that we are going to unlock that performance gap."
