Beyond that, it's not just how often the email is sent, but what the email
says, as well -- and in particular, what the subject line says -- that can affect the open rate over time...
You say they're "purposely designed" for each recipient, but if the subject line and the actual content aren't
compelling time after time, sooner or later the recipients will stop reading. And with the flood of incoming email most of us receive on a daily basis, it will probably be sooner rather than later.
Speaking personally, I know there are a whole bunch of newsletters I still receive and (very occasionally) open when the subject line seems especially interesting. For the most part, though, I just toss them unopened because I simply don't have enough time to spend reading uninteresting emails. And my experience with these has been that they aren't consistently entertaining/useful enough to bother with every time.
Targeting an email to what you perceive as a person's "interests" is not the same as making that email truly compelling. I'm not saying that's the problem in this case, just that the question needs to be asked.
--Torka