Sauber were "surprised" by their lack of pace in Monaco on Thursday as they struggled to get to within a second of those ahead of them, with Marcus Ericsson and Pascal Wehrlein comfortably slowest in second practice.
Jorg Zander, Sauber's technical chief, blamed their inability to 'switch on' the tyres, but couldn't explain why, particularly as their upgrade package delivered their best result yet at the last race in Spain, whilst further upgrades introduced this weekend have also proven beneficial.
"There’s definitely something wrong," said Zander. "It’s a little bit more work ahead of us. At the moment we don’t seem to get the tyres to work, at all.
"It [the gap] was a bit of a surprise. We came here with a new aero upgrade which works according to the data, actually quite fine. So, as expected, we thought we could draw some more potential from it.
"Of course, now while the tyre temperature management worked very well in Barcelona and panned out very well for us on both compounds, supersoft and the medium, yeah, we do seem to struggle [here] at the moment.
"The same is true of both compounds: ultrasoft, supersoft. We don't really seem to be able to put the energy into the tyres that it needs to develop the grip which is required."
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