Are you getting the frequency of your campaigns right?
Send too many email campaigns and your recipients are likely to suffer from ‘email fatigue’ and start deleting your communications without opening, or worse still, reporting your mails as SPAM. There are many, many articles on the web advising against contacting your list too frequently but not contacting your subscribers often enough can lead to the same end result. If you go for long periods of time without sending any messages, and then bombard your list for a specific event or a new product launch, you may once again trigger unsubscribes or spam reports.
Try not to let more than a month go by without contacting subscribers, either with regular content relevant to the list or with administrative requests, such as an invitation to fill out or update your preferences.
Contacting subscribers on or around a regular date each month, so that they come to expect your communications, will also help.