That's a very hard thing to answer. Depends on so many things. How you built the list, the type of traffic, how you communicate with them, how often or little you communicate with them. How your sending out your emails, service, ect.
Obviously you need to build a relationship. I kind of hate saying that because it's been said so many times these days.
You need to gain their trust, get them to like you. Get them wanting to open your emails.
Biggest things I see..
Use your real full name. And if it's hard to remember or pronounce use a pen name. Who's going to remember "John" or admin?
I signed up for a service several months ago, the guy sends emails from "Tim", really? Just about every time I don't realize who it is until I open the email. He does not make himself memorable.
You gotta email them regularly. It does not take long for people to forget you. Once they forget you, they either stop opening your emails, or mark them as spam. Plus if they stop opening their emails they will start going into the trash/spam/promotion folders/tabs and then they probably will never see them, let alone open them.
You can't go weeks or months between emails.
What do you send? Content? Promotion? Too much promotion?
What subject lines do you use? If they are deceptive or miss leading, they might stop opening your emails.
And speaking of subject lines, that is one of the most important parts, if it doesn't get them to open, well, they don't.
Emails should be consistent, your name website address, product name, or site name, ect.
Just a few ideas
If you want to PM your squeeze page I can check it out and sign up, see what's going. Not that I am an email expert, but might notice some clues