Hello @NickP373 ,
There's a myriad of reasons a contact may not open an email sent to them. It could be, as you suggested, an issue of deliverability - the contacts' domains / ISPs are filtering the emails from going through, delivering them to junk folders, bouncing the emails back, etc. It could also be an issue of the contacts simply not opening the emails - either deleting them without opening, ignoring them till much later in the week/month when they have time to check, or the emails getting lost amongst all the other ones they receive on a daily basis. It may also be that they or their email client is blocking images from coming through, which would prevent their opening of the email being tracked (more information on that in this article).
My suggestion, after reviewing your bounces and determining what to do with them, would be to read up on some of our article regarding average industry open rates, as well as best practices for getting higher open rates.
You may also wish to reach out to our Deliverability team directly to see if they have any specific insight for you, or if you'd like assistance with setting up and understanding authentication (linked to higher deliverability and open rates overall).