Charles Leclerc has said he cannot understand why Ferrari is struggling to get the most out of its Formula One car at the moment.
Leclerc finished 11th at the Spanish Grand Prix after qualifying 19th and starting from the pit-lane.
Ferrari had made changes to the rear end of his car overnight after his struggles in qualifying.
In the race Leclerc started on the hard tyre in a bid to move up the order, but pitted on Lap 17 rather than stretch out a long stint as expected.
Leclerc said his car had not been performing on that tyre.
When asked if 11th was a decent result considering where he started, Leclerc said: "No. I don't understand what we are doing wrong but we are doing something wrong and the tyres are... I went from a first hard to a second hard in the last stint, did exactly the same thing and the car is behaving in a completely different way.
"We have to obviously understand [and] work but we really need to now because it's been a few races and we are struggling with the conditions or having a very peaky car and today was no better."
Leclerc's teammate Carlos Sainz had a more productive race, finishing 5th after qualifying 2nd, but did not have the pace to hold off the Mercedes drivers.
"I put in even harder work today! It's just that it doesn't show and unfortunately this is how our situation ... it's exactly what I was talking about yesterday. We know race pace and high-speed corners is our main weakness and unfortunately Barcelona has a high degradation tyre mark, a high degradation configuration and a lot of high-speed corners which is our main weakness so that's why today we were struggling so much out there.
"And also with the predictability of the car, we were just... I did everything I could. I did the most optimal driving and stints that I could do but unfortunately P5 was the best that we could achieve."
Sainz had a more optimistic outlook to his teammate, suggesting Ferrari is starting to discover important things about its car.
"We've identified our weaknesses and we exactly know where we are lacking and the feedback is there and the intention is there, we just need time, we need to keep trying, to keep bringing things to improve the package and yeah -- Mercedes today proved that they've done a good step and it's a good reference.
"So we're going to try our best -- I see the team united, I see the team pushing flat-out in Maranello, we possibly just put the upgrade in the worst possible circuit for us so that doesn't help."