If You're Just Getting Started
Most people won't tell you why, but some will (you can boost this by giving people 5-10$ Starbucks gift cards if they give you the reason why).
Take the reason they give you, solve it, and then add that to the campaign.
Keep doing this until you have 5-10 helpful solutions for why people reject it.
Remake those solutions in different formats & wording, and resend one every week.
- Message Only
- PDF
- Video
Mix in Case Studies & Deals (as you make them up).
It will take a few months to get a good campaign, but if you work at it, eventually you'll have something that converts.