Nico Rosberg and Michael Schumacher said they both encountered problems with Mercedes' DRS wing during Malaysian Grand Prix, as they struggled to ninth and 11th on the grid.
The team had looked strong in practice, but both drivers had the wing issue at crucial moments, hampering Schumacher's final Q2 lap and Rosberg's Q3 effort.
"We had some issues with the rear wing activation today and, unfortunately, that's what compromised my last run in Q2," said Schumacher. "The car became difficult to drive, which is a real shame as it had been working very well during the earlier runs.
"The performance was much better with the wing, so of course we had to use it, and I would still say that we have made a step forward thanks to the hard work from our guys."
Rosberg said it was frustrating that Mercedes was not able to show its potential again.
"The team have made some good progress here and I had a decent balance up to Q2 today," he said. "Unfortunately, I lost that feeling in Q3, which combined with the rear wing issue, meant we weren't able to get the performance. The car is better than we have been able to show so far this season, but we don't know exactly why we are not getting more performance at the moment."
Team boss Ross Brawn agreed that Mercedes had not been good at achieving its maximum performance in 2011 so far.
"We're still struggling with the consistency of the car and are not giving Nico and Michael the best chance to show what they could achieve," said Brawn. "There are times when the car is working well and the pace is evident, however there are too many occasions where things are not functioning properly. We're quite clearly not where we want to be but will continue to work hard to achieve a more competitive position."