Open rate is a reputation score built over time. When it falls, the cause is rarely the email in front of you.
You trained the list to ignore you
A newsletter that exists because it is Thursday teaches people that your emails can be skipped safely. Send less often and make each one worth opening.
You are emailing everyone about everything
Segment by what people bought or asked about. A smaller send to the right half of the list beats a bigger one to all of it, and it protects deliverability.
Your deliverability is quietly broken
Authentication records, stale addresses and a list bought or imported years ago all push mail toward the promotions tab or the spam folder. This is a technical problem with a technical fix.
An audit of the list, the flows and the deliverability is the usual starting point. See email marketing for what that covers.
