Jenson Button says he is finding it hard to understand why his McLaren team struggled to match the pace of the leading teams in Bahrain after having started the season so strongly.
Button won the season-opening race in Australia, while teammate Lewis Hamilton secured three pole positions in a row in the first three races. The duo were never in contention in Bahrain, however, with Hamilton finishing in eighth and Button retiring with a mechanical problem.
The Briton admitted ahead of the Spanish Grand Prix it was hard to find a reason for McLaren's struggles.
"I think we have a very good car, we proved that in the first race and also in Shanghai we were very quick but I wouldn't say it is the quickest car," said Button. "If you look at the last race, it wasn't the perfect weekend for us but we basically weren't fast.
"The Lotus and the Red Bulls were a second quicker than us – I don't understand why and no one else in F1 understands knows why it is such a change-around, but I don't think they have a car that is a second quicker than us.
"I think it is circuit dependent and I also think that the conditions make quite a big difference, I know we always talk about tires but the conditions make quite a big difference as to whether you can get the tires working on a particular car."
Button also conceded it was very difficult to judge who is favorite for each race given how tight the front of the field is at the moment.
"It is very difficult to understand at the moment who is quick. You would say at Melbourne and China it was us and Mercedes and at the last race it was Lotus and Red Bull," Button said.
"I have never been in a situation where there are so many teams fighting for victories at different circuits, so I wouldn't say that our car stands out compared to the competitors and we say that you are only as good as your last race and you would say now that the Lotus and the Red Bull are the cars to beat. But we haven't had the luckiest season, either, through mistakes from myself and the team."
He added: "It is tough to try an understand. It is great for the sport and great for the fans, but for us as a team it is really difficult to understand why one why one weekend it is there and one weekend it is not.
"I think our car aerodynamically is strong and mechanically it's not too bad either but sometimes it doesn't seem to click and that must be the four things that are touching the circuit."