Testing is a difficult business, and Monday tests are the worst, especially if your problems only manifest themselves when the grip is low.
The race on Sunday lays down a nice layer of rubber, and by the afternoon of the test the next day, there is so much rubber on the track that traction is never an issue.
If you have traction problems, you have a brief window in the morning where you can replicate those problems. That window also falls when the track is coolest, which means more grip again. You can’t win.
The grip on Monday morning at Brno gave a brief window for those who were struggling with grip, riders like Danilo Petrucci, manufacturers like Yamaha. Petrucci found a small improvement in that time, falling back on a setup they used last year which helped in braking, but he illustrated the problem he faced with an example.