Email authentication is essential but there's more to deliverability. In the last few years, the ISP's and others have gotten a lot more sophisticated in that they try to determine if recipients want your email or not. It's not clear whether you're talking about day-to-day emails, your email marketing or both but here are some things to consider.
• Validate your email authentication using the SPF Survey, the DKIM Inspector, and the DMARC Inspector. I like the SPF Survey as it returns detailed error messages. People are sometimes surprised to find problems they didn't know were there.
• Engagement - The ISP's measure opens, clicks, replies, etc., to determine if people want your email or not, so if you are sending to a lot of people that aren't engaging then the ISP's (rightly) start routing more of your email to junk. Consider a sunsetting process whereby you sunset addresses that haven't engaged in a certain amount of time (90 days is common but depends on your situation). Each time you send some will likely go to the Inbox, the ISP measures engagement and routes more to SPAM. You can get this process going in the right direction by sunsetting addresses as painful as that can be.
• Check the reputation of your sending IP addresses and domains for all types of email with a service such as Sender Score. Note that sometimes an IP address can be in bad shape when you get it and so your deliverability can suffer for reasons that are no fault of your own.
• Check for blacklists with services like MX Toolbox and Sender Score.
• Take a look at your content. If you do a web search, you'll quickly find articles that summarize content to avoid. Again, this is no longer as important as it was a few years ago, as the ISP's have more sophisticated ways than sending an email to the spam folder based on content, it still does matter. It's no longer necessary to completely avoid certain content but it's good to be aware that content still counts. I'd avoid URL re-directs (even links shorteners) in email as it really does seem to affect deliverability as it appears to mask the real URL.
• Also, take a look at everywhere you're sending email as they can all affect each other. The ISP's and others have learned to identify senders by even part of an email that's the same (even if the domain name is different). Avoid practices such as sending the same (or similar) email from multiple IP addresses or ESP's or changing IP addresses frequently to improve deliverability.
• Look at what (if anything) you changed prior the problems starting.
I hope this helps.