A big part of being a successful marketer is testing. This includes your opt-in pages.
You always want to be testing at least one variation of your original opt-in page (so a minimum of two).
This will allow you to identify the variables that are causing the variations on your opt-in page.
You should be testing things like the headlines, images, call to action buttons, the offer itself, benefits, colours, etc.
When driving cold Facebook ads to your opt-in page, you are looking for a conversion greater than 20% minimum (ideally more).