After a prospect rejects your proposal, call or email to ask what didn't work. Frame it as wanting to improve your process. You won't always get honest answers, but the ones you do will show you exactly what to fix next time.
Don't be afraid of asking leads who said no to your proposal, have a follow-up call or an email and tell them like, out of curiosity to improve our processes, can you, would you please let me know what you didn't like about our proposal? You're not going to get a response every time. You might get some sugar-coated answers every once in a while, but a few of these people will tell you exactly why they did not choose you, and that would be the feedback.
