Well I think the illegal advantage they had was obviously pretty significant. Then when they lost that, they can’t just fill the gap.
They say if you stand still in F1 (development) you go backwards. If you literally go backwards, whilst others improve, you fall further down the grid.
The other problem is that aero grip is hurt when the engine has less power. The aero package is designed to be most efficient at a certain range of speeds, and if you suddenly get your engine knocked down a few rungs, you can't get to that speed, so then your aero is all out of whack. In some of these corners this year you'll see them get sudden snap oversteer mid corner. That car is a mess.
Of course the team is also a mess with their legendary strategy blunders, poor communication to the drivers, indecisiveness on the pit wall, etc.