You don't have to look any further than Trossard. By all accounts, he was a successful signing who made an immediate impact and was integral to our best run in the latter half of the season. Yet, we ended up with fewer points.
Signing players doesn't guarantee anything and often doesn't even increase your odds of anything. They're a gamble which may or may not workout. Obviously, good recruitment increases the odds of a player being a success, but that doesn't necessarily translate into more wins or more trophies for the team.
Liverpool bought Diaz whilst chasing a quadruple and he too was an overwhelming success but in the end, they didnt win the league nor the CL. It's just not the guarantee that people make it out to be.