Struggling to get the new tyres in a narrow operating 'window' in the races, Haas' boss Gunther Steiner says the team has one of the best cars in the midfield.
But he said: "Only Williams is worse than us in the race."
Pirelli's F1 boss Isola admits the new tyres were designed to deal with the overheating and blistering of the past.
As for the teams' problems, he told: "I think it's several things.
"Many teams developed their cars to handle the tyres more carefully, together with our efforts to reduce the risk of overheating.
"On the other hand the situation is not so bad. In Baku, the temperatures in qualifying were extremely low and the laptimes were still very fast. So you can't say that the tyres do not produce any grip," Isola insisted.
He also denied that the operating 'window' of the tyres is now too small.
"No, we are always working to enlarge the operating window," said Isola. "I do not think these tyres work in a narrower range than last year's. The window has shifted only slightly.
"We are also being more conservative with our selection of compounds in order to allow the drivers to push harder in the race.
"There are now a few more problems with the warm-up, and the first lap out of the pits may be a bit difficult, but then the tyres work well," he said.
"If you look at the data, you will not see any signs that something is wrong."
As for why a team like Haas is struggling so significantly, Isola answered: "The tyres are still quite new for everyone. That's why it's normal for some teams to have more problems than others.
"We have to adapt our product every year because the cars change every year, but during the season, everyone learns. That's why the differences are a little bigger now than they will be at the end of the year."
