Red Bull Racing's Max Verstappen and Pierre Gasly both struggled for grip in Bahrain on Friday when their RB15 was shod with Pirelli's soft tyre.
Verstappen clocked in sixth in FP2, the cooler evening session which was more representative of qualifying and racing conditions, leaving the Dutch driver to ponder the reasons behind his car's disappointing behavior on the C3 rubber.
"It has been okay but there is still quite a bit of work to do," said Verstappen.
"I am still not happy with how the car is behaving. We will look into the data tonight to see what we can improve.
"On the medium tyre, we were really competitive. We should have been fast, but out of the last corner I had a little issue so I lost a few tenths there.
"But then on the softest tyre, I didn't feel the grip like I should have. We need to look into that."
Gasly's lowly P12 result was perhaps even more concerning, the Frenchman concurring with his team mate on the RB15's tyre issues.
"FP1 was good. FP2 was good as well, but with the soft tyres we tried something and it didn't really work," said Gasly.
"We are still lacking a bit of pace with the soft tyres, but we will work tonight to find out for tomorrow."
Speaking after the evening session to Sky F1, Red Bull team boss Christian Horner was unfazed by his team's relative under-performance.
"These times don't tell us much about our speed," said Horner. "We missed our 'window' on the soft tyre. More is coming tomorrow."