This happens often in email marketing, and there are explainable reasons that could be the problem (not definite).
1. Check your list:
Step #1: Clean your list, remove bounced emails, and invalids.
Step #2: Segment your list; (1) group inactive and active subscribers differently. (2) further segment your active subscribers to group engaged and unengaged subscribers (30-90 days benchmark is best).
Step #3: Send marketing emails to only your active and engaged subscribers. (This will dramatically increase your open rate, all things being equal)
Step #4: Retarget your inactive subscribers and re-engage your unengaged subscribers.
2. Check your domain reputation: Make sure your DMARC configuration is still in tact. If your reputation is low, change your sender IP or domain entirely.
3. Optimize Your Subject Lines: Bear this in mind;
a. Your recipient only sees the first few lines to a maximum of about 30 characters.
b. Choose words that elicit curiosity to begin your subject line. Add more intrigue with your preview text.
4. Pay attention to your audience and send relevant content
If you need more explanations you can dm me. Let's dig deeper.
Kind regards.