My guess is they probably messed up the setup and probably went too light on downforce. Lewis had massive understeer in the corners (hence his comment about the car not turning) and a loose backend on corner exit (the latter of which is a direct consequence of their lack of rear downforce).
The Monaco package wasn’t meant to be bringing a huge performance boost but rather make the car more compliant and predictable.
The Silverstone package (based on early season development) should hopefully bring actual performance gains. They haven’t really upgraded the underbody all season (only floor edges), and they desperately need a more efficient rear wing and probably new front wing as well. Assuming the upcoming package will focus on those areas.
I do wonder whether they’ll bring new sidepods too. Always got the impression that the Monaco sidepods were simply a stopgap.