We have sent out two campaigns which we plan to do monthly. The issue is that some users received the first one in January but not the second from February. It is not in their spam.junk mail either. Constant Contact says it was sent and received and it is counted in our numbers for reporting.
Why would a large number of emails be successfully sent to one month but not the other and how do we fix it?
That does sound like an unusual problem. There's a few things we would need to look at, and since our system isn't showing a bounce, we would need to rely on what your recipients are telling you. Are the majority of these emails in the same domain? Are they managed by a corprorate IT team or running through the same firewall by chance? Those are the questions that seem most fitting for what you've described so far, if you have some examples and want a speedier resolution, it may be best to contact our Deliverability team in the Account Review department directly. You can reach them here.