There are lots of variables that impact your open rates. Here's some that are common... as well as how to deal with them.
Do people recognize you?
Oh. My. Gosh. I cannot tell you the number of emails I receive where I don't know who the heck it even is that's emailing me and I delete it under the assumption that it's junk. I bet you do the same!
Make sure you are utilizing a sender name that they can remember - and no, don't just set it up to send as "Susan"... because there's a gazillion Susans (sorry if your name is Susan).
All of my emails are sent out as Kimi Kinsey!
Subject Line Quality
Of course this is a big one. If your subject lines suck, no one is going to want to open up your emails. While there's more to getting people to open up and read, your subject line is the place to start.
Is your list huge?
Like, I'm talking in the 50k+ range. If that's you (first off... mad props because you're awesome!) you probably have a lot of dead weight - a bunch of subscribers who haven't interacted with you in a long time (and it maybe isn't even your fault).
You'll want to be sure to work on warming up your cold subscribers, or cutting them from your list entirely.
How old is your list?
If you've had your list for yeeeeaaars, chances are you have a lot of people on your list who are no longer interested in whatever you're emailing about.
-Or- Maybe you haven't emailed them in a while, so your emails are flopping because they don't remember who the heck you are.
Did you change direction in your business?
I can personally attest to this one, because I've changed direction in my business a few times... and it's definitely meant that I needed to put in the work to re-qualify my audience and make sure I was emailing people who were interested in my new ventures.
You'll have to do the same - even if it means reminding them that they can unsubscribe. It's maybe an unpopular opinion, but it's better to have someone unsubscribe than to continue paying to have them on your list if they have no interest in your offerings - especially if they aren't buying anything.