What’s the hardest part of turning a client request into a signed project?

For those involved in pre-sales, consulting, agencies, software projects, or client services:
What’s the single most frustrating part of going from a client’s request to a signed project?
I’m not looking for generic answers like “finding clients.”
I mean after an opportunity already exists.
What consistently slows things down, causes mistakes, creates stress, or hurts profitability?
Curious what people have seen across different industries.