Christian Horner says Red Bull is not able to show the true potential of the RB11 because the car is always in "a compromised state".
Red Bull was the only team to win races other than Mercedes last year as Daniel Ricciardo took three victories on his way to third place in the drivers' championship. The team itself finished second in the constructors' and was confident of closing the gap to Mercedes this season.
However, problems with the Renault power unit have hampered Red Bull so far, with Ricciardo's sixth place in Bahrain equalling the team's best result of the year as it sits fourth in the constructors' championship. Horner says there has been a clear improvement with the RB11 but the issues faced on the engine side have masked its potential.
"We are definitely making progress with the car," Horner said. "[Bahrain] is probably our most competitive weekend. The problem is we are running the car in such a compromised state that we are not in an optimum window with the car.
"Our top speeds are down, then you end up trimming wing out to try and get somewhere sensible, then that hampers things like tyre warm-up and downforce. We are not in a window with the car where we want to be, but made quite significant progress."