First you want to know that the people you are sending to that aren't opening or clicking are actually receiving your emails in the inbox.
A good list cleanse company can help identify if there are any issues with the emails on your list - and we have learned a ton going through this process - like spam traps will open your emails! So just because you're seeing an open doesn't mean anything.
An open is the most unreliable metric in email marketing. We like to look at it- but in reality, all it means is that the images were downloaded. And some ISPs cache images.
What is reliable is the click.
We have done this to pretty great effect:
Set up a sequence of 3 or 4 emails asking for a specific action - to register in a Webinar or download an attractor (aka lead magnet). Each email in the sequence asks for the action differently. Different subject line, different message.
Key: you must know your audience and the content must be relevant.
If someone doesn't open (or all they do is open) they get the entire sequence. As soon as they click or take the desired action on the landing page, stop the sequence. In fact you could have another sequence of emails that you trigger leads into that clicked but didn't convert... Because you can talk to them differently.
You can be more personalized in your sends. At the end you see how many of your non openers have opened.
And you start again with the non openers. We did this where we'd only send a series like this once per month to the non openers. We always added more to the openers and as we sent more campaigns to the openers we got more who click
And those who are opening but not clicking you work to find their hot topic - and those that click, then you can dive in with more content that is helpful based on their click.
I think it's something like 80% of people will buy after the 7th touch (I think it depends on your market), but most people stop after the third try.