I always compared the current Mercedes situation with Renault's 2007 season.
And I agree with ringo; the car's conception is maybe the problem.
There is many similarities between the problems faced by the R27 and the problems that the W01 faces today...
The Michelin tires required a lot of weight on the rear tires, for the Bridgestone it was the opposite. In 2007, Renault did not allow enough weight to the front tires. Same story for the 'downforce' distribution... that's why that car lacked of grip and aero efficiency.
I think Mercedes did similar mistakes in 2010; the narrower front tires do not need as much ballast as before. And Mercedes changed its weight distribution twice this season to rectify that, but I think it was insufficient; the car is too short and compact... and it's more difficult to rethink the aerodynamic due to the new drastic regulations (homolagated nose etc.)
Same problem for Renault in 2007, but in the other way around; they were urged to move more weight toward the front and modify the FW and the nose in order to generate more downforce. But Renault discovered the problem too late in the season and unfortunately the wind tunnel and the budget were very limited...