Odds are your agent is failing. You can perform a couple of simple tests to verify this. First you can look through the log.nsf on your server. Find the page containing the time period when the agent runs and see if there is an entry for the failure. You can also manually run the agent w/ script debugging turned on to see if it fails.
If it’s not the agent the next step is to manually send an email to the group in question from your email account. You can then look through the log.nsf to see if your mail document is encountering any errors.